The mechanical arm seizes envelopes and shoves them onto a conveyor that moves relentlessly in front of Patricia Johnson's keyboard on the midnight shift at Washington's cavernous main post office.

At a rate of 50 letters per minute, Johnson, in a split second, must remember all the thousands of addresses within the 20017 and 20018 zip codes of Northeast Washington and must punch, by memory, the proper two-digit code to route each piece of the morning's mail to one of 70 letter carriers.

In the past, supervisors would measure her performance by watching her or examining a batch of her work. But now, her supervisor sits at a video screen, punches a computer keyboard, and can immediately see how many mistakes Johnson is making. If her accuracy falls below 95 percent for an extended period, Johnson can be fired.

"The mail is running by me and running by me, and the machine kind of hypnotizes you," Johnson said, "and this computer is looking over your shoulder, watching you. It gets very stressful . . . . The supervisor knows everything about you, right in that machine."

Johnson, 30, the single parent of an 11-year-old boy, is one of a growing number of American workers who no longer labor solely under the watchful eyes of the boss or the foreman. Now, millions of blue-collar and white-collar employes are working under the even more watchful and relentless scrutiny of the computer.

Computer monitoring -- the timing of a worker's speed, efficiency and accuracy -- is being used in a rapidly increasing number of work places and is beginning to spark intense controversy in some of the nation's key industries.

The office worker, the telephone operator, the supermarket cashier, the bank teller, the insurance claims processor, even the waiter, waitress and the hotel maid -- and some managers and supervisors themselves -- are often linked to computers that can time and record virtually every movement -- down to the number of keystrokes per hour hit by a typist.

Computers can provide management with unusually detailed performance reports -- monthly, weekly, even hourly -- that not only tabulate an employe's speed, but also compare it with his fellow workers' or with his office average or even with his past performance.

Some computer systems also keep track of how many times the employe leaves the work site, for bathroom visits or other breaks, and can monitor how long the breaks took and how long the worker actually spent working.

In many cases, computer-generated performance reports are being used to structure "piece-rate" pay systems and to form the basis for disciplining, demoting or even firing the slow or under-skilled employe.

Large businesses and computer firms tout computer monitoring as an effective means of improving productivity, speeding the flow of information and giving customers faster and better service. Precise standards, they say, also provide an objective measure of efficiency that many workers welcome, especially if the system is used for rewards such as merit pay.

But critics contend that computer monitoring represents the intrusion of "Big Brother" into the work place, a development that they say increases stress dramatically and may cause illnesses such as hypertension, heart disease, migraine headaches and stomach maladies among workers who suffer "technostress" from having a computer constantly watching and from being evaluated by a sometimes unseen supervisor.

Moreover, opponents of monitoring say that computer-paced employes often provide poorer, slower and even less courteous service to customers because they are forced to work in an "electronic sweatshop" where speed is at a premium, according to labor organizations, academic critics and other skeptical observers of computer-tracked work places. Huge Investment

American business is spending an estimated $50 billion a year on computerization of the work place, and it is estimated that by 1990, 40 to 50 percent of American workers will use computers, according to Arthur D. Little Inc., a management consulting firm.

Almost any computer system can be adapted to monitor employe output, but it is unclear how widespread the practice has become. One indicator, a recent nationwide survey of video display terminal operators, showed that 35 percent were monitored by computer, according to 9 to 5, the National Association of Working Women.

The use of computer monitoring has quickly spread to a surprising variety of work places in the past few years. A sampling of some current applications:

*At Washington's Ritz-Carlton Hotel, when a maid enters a $650-a-night suite to make up the room she must first punch in a two-digit code on the room telephone, informing the central computer downstairs that she has entered. Another two-digit code must be punched when she is finished. While the new system tells management when each room is ready, it also provides a detailed daily log of each maid's speed and a record of her movements for an entire day -- a system management likes and the maids often strongly resist.

*At AT&T Communications, a giant computer system keeps track of each telephone operator's AWT, or "average work time" -- the time the operator spends per call, usually about 30 seconds. If an operator consistently takes longer than most colleagues, even a few seconds longer, the operator can be disciplined or dismissed, a practice that is vigorously opposed by the Communications Workers of America.

*At Northwest Orient Airlines in Minneapolis, the 55 data-entry workers who feed ticketing and payroll information into company computers are expected to type at a speed of between 9,000 and 16,000 keystrokes per hour, with a sophisticated computer system keeping track of speed. Fast workers can win the right to have an hour of "flexitime" in setting their schedules, but slower ones can lose pay. All workers must maintain a speed at least 75 percent as fast as the three fastest workers, or they can be fired.

*United Parcel Service managers in Maryland and Virginia, after studying delivery routes, feed the driver's daily reports into a computer that considers the number of stops made, packages delivered and miles traveled, and computes how long the route should have taken. Performance is tabulated daily in hundredths of hours. UPS does not use the system for disciplinary purposes, a spokesman said, but the system nonetheless prompts strong opposition from the Teamster drivers. "The supervisor knows everything about you, right in that machine," said Patricia Johnson.

*At Giant Food stores, the space-age optical scanners that read the codes on each item not only provide quicker checkout and inventory control for the company, but also track the cashier's speed. Giant gets computer printouts, which it posts in the work place, showing the number of customers, food items and dollars rung up by each cashier. Initially, cashiers' names were posted with their ratings, but protests from the supermarket union were so strong that cashiers are now identified by numbers only.

*At the Social Security Administration's regional data-processing center in Wilkes-Barre, Pa., about 1,000 federal workers, almost all women, are expected to meet quotas as they punch their computer keyboards -- for instance, 138 forms per hour for those entering annual wage forms of SSA recipients. The computer keeps a running total on the screen of forms completed, and it can compute the amount of time actually worked by the employe that day.

Time-and-motion studies of industrial workers are nothing new in the American work place. In the 1890s, industrial engineer Frederick W. Taylor meticulously observed the movements of laborers hauling pig iron and factory workers toiling on assembly lines. He perfected a method of stopwatch observation of workers' movements in a process that came to be called Taylorism, which evolved into the modern concept of "scientific management."

But the computer and its programs have far surpassed Taylor's stopwatch in giving management a virtually unlimited capability to assess each worker's productivity in minute detail.

The computer opens vistas of ultra-accuracy in dissecting the work process. Efficiency experts measure employes' movements in TMUs (time-measurement units) that can compute, for instance, how long it takes a typist's brain to refocus her moving eyes on a page: approximately 7.6 TMUs. One TMU equals 0.00001 hour, or 0.036 second.

"There are now ways to analyze work performance that are even beyond human comprehension, and so you can be held accountable to a standard you can't even comprehend," said Robert Ellis Smith, author of "Workrights," a study of the clash between employe and employer on issues of individual rights.

Harley Shaiken, a researcher at the Massachusetts Institute of Technology who is a leading authority on computer monitoring, has described some of its applications as "an electronic straightjacket." Shaiken, in an interview, said, "People have been time-studied in the past, but no one had the capacity to be time-studied 24 hours a day, 7 days a week . . . . The pervasiveness of computer monitoring really makes it a new issue." Fear of Punishment

"Some people find it really terrifying," Shaiken said. "The very existence of monitoring apparatus serves a disciplinary purpose" because of the fear of punishment.

Although postal workers, for example, are rarely disciplined as a result of monitoring, the potential is frightening to many workers, said William Burrus, executive vice president of the American Postal Workers Union. "All you have to do is fire one employe in a 5,000-employe office and you have 4,999 others who are working scared," he said.

From management's standpoint, much of the opposition is misplaced and should be directed more at people who misuse the systems rather than at the systems themselves, said Vico E. Henriques, president of the Computer and Business Equipment Manufacturers Association, a group of more than 40 firms with more than $125 billion in annual sales.

Managers who set reasonable standards, use discipline fairly and offer some form of incentive will elicit employe cooperation despite initial resistance to computers, he said, while those who design repressive monitoring systems will encounter trouble.

"I don't think the effect of machine counting is any different than the general problem of good employe relations," Henriques said, "If you have somebody standing over you and threatening and saying 'Work harder! Work faster!' it is intimidating. But a person is more intimidating than a machine. A person can fire you," while a machine only reports your scores.

Although the issue has not yet been fully studied, preliminary research suggests that the health effects of computer monitoring can be severe, according to Michael Smith, an industrial psychologist at the National Institute of Occupational Safety and Health, the government's job safety research arm.

"It is quite clear from the literature that employe monitoring by computers creates a dehumanizing work environment in which the worker feels controlled by the machine," Smith wrote in a recent study of video display terminal operators. Smith said in an interview that the machine-monitored worker often feels a "loss of control" that is a key cause of stress.

Working under pressure in such settings causes increased blood pressure and secretion of adrenaline and cholesterol, heightening the risk of heart disease, Smith said. "You get more cholesterol out of that stress reaction than out of eating 10 pounds of cheese." High-Tech Tensions

Patricia Johnson came to the Washington post office 12 years ago. She has long since mastered her job, she believes, and she usually has an accuracy rate of about 99 percent at the 50-per-minute pace of the letter-sorting machine. But as she sits in a row of 12 mechanical sorting machines -- with a supervisor at the video screen monitoring her and the other operators -- the tensions can be maddening, she said.

"The noise is incredible . . . . A lot of people wear their Walkman radios . You have hundreds of people, and tremendous noise, and tremendous heat. The air-conditioning breaks down quite often too," she said. "And it's like you are strapped into that machine. You gotta have your rhythm. Rhythm is everything. The average employe is a good keyer, but you can become hypnotized. Those machines can hypnotize you."

Adding to the tension, she said, is uncertainty about whether the supervisor is scrutinizing her work. The worker never knows, she said, whether the supervisor is watching, or just what he is watching. "Without getting up from his desk," she said, "he can see what you are keying in, whether you missed a character or only keyed in one digit instead of two. Or if you miss the hit altogether" as the envelopes whiz by.

"People are human and you have all different kinds of supervisors," good and bad, Johnson said. "But it's like overseers sometimes, watching and walking and walking and watching."

Carl H. Robinson, president of the 2,500-member D.C. Local of the American Postal Workers Union, said employe failure to keep up the required 95 percent accuracy rate results in about five dismissals per year, which are carried out only after attempts at remedial training. But sometimes, he said, the Postal Service is heavy-handed in failing to consider the "human aspect" of why a worker has trouble: "They're only concerned with productivity."

The Postal Service, with its abundance of acronyms, calls its monitoring systems EDIT and EZR -- the Engineered Data Isolation Technique and the Electronic Zip Retrofit -- and uses them not only to monitor employes but also to monitor machines, said Postal Service spokesman George Conrad.

Postal officials, citing employe tensions relating to the current stalemate in contract talks, would not allow interviews with Johnson's supervisors or discuss her specific complaints, but Conrad said the system is intended to monitor performance fairly and generate constructive criticism.

Operators may be using poor hand position or making other correctable errors that the computer will help detect, Conrad said. With retraining, he said, many correct their deficiencies. Sometimes, he said, supervisors find that the mistakes may be made by the machines rather than the operator.

Regarding employe resentment of the system, Conrad said some opposition is inevitable but that the system is crucial in monitoring production: "I can't speak for how people feel about this. I am sure in any kind of production job there are different ways of seeing what we are doing."

Tomorrow: Conflict over computers