Computer networks now have almost boundless applications: Salespeople on the road use them to inform headquarters of the day's successes and failures. Auto dealers use them to order spare parts. Horse breeders use them to ferret out pedigrees of particular mounts, and doctors use them to get help in diagnosing tough cases.

Newspaper reporters send in stories to their offices through them, while homebound or disabled people electronically "chat" to friends next door or a continent away.

The basic tools of a network are a computer, a telephone and a subscription at one of the proliferating companies that provide phone-line link-ups from one computer to another.

In the 1980s, the computer networks have burgeoned into a $4 billion-a-year industry, nourished by a special exemption on interstate phone fees that the Federal Communications Commission granted in 1983 to promote such growth.

Now the FCC has proposed taking back that break, and the industry is livid. An electronic call to arms has flashed across the networks' screens, pulling together the diverse collection of institutions and individuals who use and operate them. The result has been a flood of protest letters, phone calls and legal briefs to the commission and to Capitol Hill. FCC staff members call the outpouring the biggest in memory on an agency issue.

The FCC says its proposal would bring the charges for computer network users in line with what ordinary telephone subscribers pay for access to long-distance lines. The FCC says its plan is a matter of simple fairness..

But opponents, call it a gross infringement on their rights and a potentially disastrous blunder that could drive small companies out of the business of providing the networks and strangle the development of the industry.

Users dial local numbers of the link-up companies, and through phone lines tie their machines to computers there, which in turn link them to other computers or to data bases.

By typing commands on their keyboards, users can get wire service news reports, stock prices, credit verification, horse pedigrees, airline schedules, retail bargains and advice on farming and health.

Through so-called bulletin boards, users can send personal messages to others on the network. If they want to analyze data but don't have the computer program necessary, they can temporarily use the program they need through a network.

In 1980, there were about 400 commercial data bases in the world, according to Tymnet, a major link-up service operated by McDonnell Douglas Network Systems Co. By 1986, there were 2,901 worldwide. In the same period, industry sales worldwide grew from about $1 billion a year to $4 billion, Tymnet estimates.

This type of service is considered key to the "information age," that oft-heralded time in which every home and office will have a computer terminal and through it access to just about any information anywhere in the country and, theoretically, in the world. But to be universal, it must be affordable.

And that is why the FCC decided in 1983 to exempt network operators from "access charges" assessed on long-distance telephone companies to compensate the local phone companies at either end of a call for use of their facilities.

The exemption has helped keep computer network rates low. For instance, in off-peak, nightime hours, Tymnet charges customers as little as $1 per hour for basic connection services. Full rates during peak hours are considerably higher, though by no means out of sight for corporate budgets. Tymnet's run to more than $11 an hour. Typically, users also pay a fee per character transmitted and, if it is commercially operated, a fee to the data base they are using.

The FCC maintains that operators and users have known all along that their exemption was temporary and should have been planning for its termination. The exemption amounts to a subsidy, says FCC Chairman Dennis Patrick, and in the long term, he argues, subsidies discourage development of efficient and healthy industries. "If it is not paid by one group of users of these facilities, it is paid by another," he said.

Sending information by voice or computer across state lines is essentially the same thing, proponents of the charge contend. They also hold out the possibility that charging the computer users could work toward lowering long-distance rates for conventional telephone calls.

The industry, meanwhile, says it will simply pass the new charges on to the user. By most estimates, that would mean a markup of hourly rates charged to computer users by more than $4.

In its counteroffensive against the FCC, the industry has found plenty of hard-luck cases, people earning little money or doing good works who use the systems mostly in off-peak times and would possibly be forced into electronic silence.

There are doctoral students and medical researchers. There is a blind user in Independence, Mo., who distributes music he composed and fiction he wrote through the networks. There is a deaf and dumb veteran in Miami who says he enjoys communicating with people through the networks.

Dr. Robert Fusco, a gastroenterologist at a small private hospital in Sewickley, Pa., uses computer networking to get abstracts of medical literature and for help in diagnosing specific cases. One service newly offered by the American Medical Association, for instance, asks a doctor for specifics of a patient's physical characteristics, test results and symptoms and then offers possible diagnoses.

Fusco says the computer helps make him equal in resources to his colleagues in the big cities. "I have the same information they have if I can tap into the data base, without actually being in a medical center. That's the beauty of it."

But he says he fears rising costs. "If you add $5 an hour to it, the hospital is going to say, 'well, that's a little expensive.' "

Small companies argue it would be another unneeded added cost in generally hard times. One case in point is the Pig Improvement Co., a Kentucky firm that employs about 180 people in the business of breeding pigs and selling them to farmers. Its salespeople use Tymnet to communicate with the home office; the company also offers an after-sales service by which about 250 customers log into a company-maintained data base to get information on caring for the animals.

Tymnet has told the company that under the new rates, its bills would rise by about $1,000 a month, according to company data processing manager Jeff Brown. "We provide the service free right now," he says. "So obviously we would have to start charging," or at least think about it. One other option the company is considering is dumping the service and perhaps handling customer inquiries the old way, by voice, through a toll-free 800 number.

But most users are large corporations. Tymnet, for instance, estimates that only about one-fifth of its users are individual or residential users tapping into consumer-oriented data bases. And about two-thirds of its customers pay more than $2,000 a month for Tymnet services.

Tymnet says that one of its customers, Westinghouse Electric Corp., whose employes generate about 450,000 computer messages per month, would find its communications costs rising by about $1 million a year and would respond by reducing service to many areas.

The industry also argues the fees would create new incentives for companies with big-volume flows to single destinations to establish private phone networks. The FCC is generally opposed to such private lines, on the grounds that local public phone companies need a maximum number of customers to prosper and further the goal of "universal service."

Overall, the industry says that the rates would be a supreme case of short-sightedness. Computer networks have the potential to drive overall economic development, it is argued, by making sophisticated data, analysis and communication available to every company, large and small. Higher rates would cut back the growth of network operators, the industry maintains, and make many clients bail out to find other ways of communicating.

Network operators say that other fees assessed on them assure that they're paying their fair share. And they contend that the additional money the FCC proposal would extract is simply too small in comparison to what long-distance companies already pay to bring down long-distance rates in any meaningful way. Telenet Communications Corp., another major U.S. network operator that is fighting the idea, says the effect on lowering long-distance rates would probably be less than 1 percent.

The industry is trying to use its technology to fight the FCC's plan.

The Columbus, Ohio-based CompuServe network has put some special items on its "menu," the list of possible functions that users see on their screens when they sign on by telephone. Item 9 is "FCC proposal information." No. 10 is "Send an FCC Congressgram." Users who request those functions with a keystroke or two find a plea for help flashing across their screen.

"We urge you to join us in protesting this costly and onerous measure by writing directly to Congress and the commission," the message says.

By other commands, users can get a sample protest letter on their screens and addresses of members of Congress and commission members. They can write a letter on the screen which, for a 50-cent fee, CompuServe will have printed out and put in the mail.

As a special break, the on-line time they spend writing that letter is free. Already about 5,000 letters have been generated this way.

"We've never really used the service in an advocacy type role before," said CompuServe spokesman David Kishler. It is now considering including a permanent data base for letters to Congress on any subject, he said.

FCC Chairman Patrick said he expected protests but has been surprised by their strength.

No decision has been reached on whether to go ahead with the proposal, he said. Some commission staff members, however, speculate it will be modified to allow for a phased-in implementation, or may even be dumped altogether. When the decision will come is unclear. The original Jan. 1, 1988, implementation date is not considered feasible