PHILADELPHIA -- When Carol A. Leisenring left President Reagan's Council of Economic Advisers three years ago to become CoreStates Financial Corp.'s chief economist, she stepped into a traditional role.

She prepared forecasts, made speeches about the direction of the economy and met with clients of the Philadelphia bank holding company.

But that emphasis changed in March, when she began to lead strategic planning at CoreStates in addition to her duties as economist.

Rather than just giving the outlook for gross national product or retail sales, she is using her Ph.D. and skills as an economist in planning for her company's future on what she calls the ''nitty-gritty business level,'' such as how a new product or business line would fit in with existing operations.

Leisenring's experience is not rare. No longer are big corporations keeping large staffs of economists merely to try to divine the direction of the economy.

Instead of providing forecasts to marketers and planners, corporate economists now are becoming the marketers and planners.

In part, the change is a matter of survival. Many corporate economics departments have disappeared in recent years because of cost-cutting that brought all staff functions under intense scrutiny. Remaining economists are being pushed into decision-making roles in line management jobs rather than filling only advisory roles.

''This is not a growth industry,'' said Kenneth T. Mayland, former economist at First Pennsylvania Bank in Philadelphia who now is chief economist for Society National Bank in Cleveland.

Bank economists may have been hit hardest as banks have merged and eliminated redundant support departments. Mayland left First Pennsylvania when it was considering a merger with Marine Midland Bank, and he wasn't replaced. (First Pennsylvania since has been taken over by CoreStates.)

But axes are falling elsewhere. For example, the huge Eastman Kodak Co. early this year cut its economics staff from five to one.

Though the functions of corporate economists may be changing, there is still plenty of demand: The number of professionals who call themselves business economists has increased, according to the National Association of Business Economists, based in Cleveland.

''Their location in the firm is changing,'' said David L. Williams Sr., executive secretary of the association. Instead of having large, centralized staffs, many companies put an economist in each division to support line managers, he said.

The National Association of Business Economists has 3,800 members, up from 3,600 in 1989 and 3,400 in 1988. Though 1990 salary data aren't fully analyzed yet, the median base annual salary of a business economist will be close to $65,000, Williams said.

In the corporate world, economist jokes have become cliche. Most jokes poke fun at the alleged inability of economists to predict correctly, offer a firm prediction or agree with other economists about their predictions.

''A number of people are afraid to say they're an economist,'' said Gary J. Stevens, president of Stonehill Management Consultants Inc. of New York, one of the main search firms for economists.

Economists are calling themselves ''business analysts, market researchers'' or ''quantitative analysts,'' Stevens said.

Even economists acknowledge that there are reasons for the profession to be embarrassed. Some economists in the 1970s and early 1980s ''oversold'' their ability to use elaborate computer simulations to predict the economy a year or more away, said Leisenring.

Their shortcomings became apparent, she said, when interest rates shot up to unprecedented levels in the early 1980s and the computerized simulations suggested that ''the world should have come to an end.''

Not only were many forecasts unreliable in that period but corporate executives became disgruntled because staff economists could not say what forecasts meant to companies' business lines.

''In many organizations, the economics function has not played a significant role,'' said longtime forecaster Lawrence Chimerine, who recently left the WEFA Group of economic consultants in Bala Cynwyd, Pa., to go out on his own. ''It's the issue of relevance. Too many of us over the years have spent too much time looking at the economy without looking at the relevance to business.''

Financial forecaster Paul Getman of Regional Financial Associates in West Chester, Pa., agreed. ''It was partly our fault as a profession that we were not giving management the information they needed to make intelligent decisions.''

Business economists have adapted, putting their focus on how to improve the sales and earnings of their companies, said James Smith, a former Union Carbide Corp. economist who teaches at the business school of the University of North Carolina at Chapel Hill.

''And if they don't make a bottom-line contribution, like other middle management they're gone,'' said Smith of the National Association of Business Economists.

Economic-forecasting firms such as WEFA and DRI/McGraw-Hill in Lexington, Mass., have had to adapt, too. When a company wipes out its economics staff, its consulting firm loses its client and its only contact at the company.

So, as the job of corporate economist gets redefined, the consulting firms try to find new ways to make themselves useful.

Some economists wind up as consultants to the corporations that used to employ them.

''You see a lot of us doing the same thing in the service sector rather than in the manufacturing sector,'' said John H. Qualls of St. Louis, a former Dow Chemical Co. economist.

In late April, for example, American General Corp., an insurance company, hired Qualls as part of its effort to fend off a proxy takeover battle by Torchmark Corp.

Qualls studied the impact, or value, of American General's operations to the economies of Nashville, Houston and Evansville, Ind., three cities in which American General operated. His analysis suggested that if American General pulled out of Evansville, 3,000 jobs would be lost and the local unemployment rate would rise to 7 percent from 5.8 percent.

At CoreStates, Leisenring has found the economics and strategic planning functions ''a good blend of responsibilities.''

''If the economist is doing his or her job, he or she is identifying things down the road that will impact on the company,'' she said. Leisenring has company at CoreStates, a colleague who can appreciate the value of economics training to a large banking organization.

Terrence A. Larsen, who holds a Ph.D. in economics, did a stint at Chase Econometrics, one of the two firms that merged to form WEFA, before joining CoreStates's Philadelphia National Bank 13 years ago as the bank's economist. Today, he is CoreStates's chairman and chief executive officer.