With the unceremonious ouster of Postmaster General Paul N. Carlin and his replacement by former American Airlines president Albert Vincent Casey, the U.S. Postal Service Board of Governors has once again turned to the business world in hope that a proven corporate manager can work private-sector magic on one of the government's largest, oldest and most sluggish agencies.

Whether Casey can restructure the old Post Office Department to compete in a vastly different world may well determine what role the Postal Service will play in the next decade.

The changeover comes as the Postal Service enters what could be its most difficult era. Private competitors such as air freight companies and courier services are showing that they can deliver packages more efficiently, and at a profit. The growth in electronic mail service and information technology, with the specter of home electronic banking near, threatens to make much of the traditional role of the mail carrier obsolete. Postal observers are asking whether the Postal Service will go the way of the railroad, which reached its peak in the immediate post-World War II years.

"Can what happened to the railroad happen to the Postal Service? Absolutely!" said Michael F. Cavanagh, a private consultant on postal affairs. "Can the Postal Service prevent that from happening? Absolutely not."

In the meantime, Cavanagh said, "the best thing it should do to forestall that changeover is to keep first class postage as low as possible."

The Postal Service's official historian, Rita Moroney, counted 11 postmasters general who have been brought in from the corporate or business world. They include President Dwight D. Eisenhower's choice of automobile dealer Arthur Summerfield in 1953, and former American Can president Elmer T. Klassen during the Nixon administration in 1972.

Said Moe Biller, president of the American Postal Workers Union, "It's been a feeling that the great tycoons of private industry were going to do a great job for us. But private industry hasn't done that great a job in the United States over the last two decades. Look at the steel industry and the old smokestack industries."

The sense of a Postal Service facing its own destiny permeated yesterday's news conference, at which the chairman of the postal Board of Governors, John R. McKean, introduced Casey, the 66th successor to Benjamin Franklin.

Asked why Carlin, a 16-year postal veteran, was dumped after 12 months on the job, McKean referred vaguely to "a changing environment that required new market directions." He said the Postal Service needed to be reorganized to meet the new challenges and he implied that Carlin, as a veteran of that system, was not fit to do the job.

"If you believe that a government bureaucracy can adjust itself and reorganize itself," McKean said, "then you believe in monogenesis."

Casey himself was flip and caustic about his own intentions to meet the challenges, saying he only intended to stay "if I'm really good, six months," and saying his toughest challenge would be "convincing my wife to accept the Washington scene." He refused to answer serious questions seriously.

One scene seemed symbolically to capture the new dynamics of the Postal Service management. McKean said that with Casey on board "you can expect some disinvolvement by this board in day-to-day decisions." But it was board Chairman McKean, the most powerful figure on the panel, who held the podium for 20 minutes, fielding tough questions on major postal issues while Casey stood by smiling.

"McKean's holding all the cards over there," said one knowledgeable postal insider, who asked not to be quoted by name. "I think he's the power behind the throne."

Carlin was fired 12 months into the job primarily because he did not move fast enough in implementing the board's philosophy of turning the Postal Service into a leaner, more efficient agency. That would have entailed streamlining the bureaucracy and immediately scrapping some costly blunders from the past, such as the ECOM electronic mail system.

The problem of the work force illustrates the conflicting demands placed on anyone who serves as postmaster general, who is the chief executive officer of the nation's largest unionized employer.

A Postal Service job traditionally has been considered a secure, high-paid position with good fringe benefits and pensions. For most of its life, the former Post Office Department was able to rely on generous government subsidies to cover inefficiencies.

But since a 1970 reorganization, it is now a business that must keep an eye on the balance sheet.

The labor-intensive U.S. Postal Service now employs 740,000 workers, and about 8 of every 10 cents it collects go to payroll. The Postal Service headquarters at L'Enfant Plaza employs about 3,100 people.

The mail delivery system also labors under four layers of bureaucracy: the national staff here in Washington, regional staffs, district staffs and individual postmasters around the country.

McKean said yesterday that he expects staffing reductions to come by streamlining all four of the layers and substantially increasing the power of lower-level field managers to make decisions without being second-guessed from Washington.