Greyhound has left the driving to someone else. Singer makes missile components instead of sewing machines. American Can has canned cans in favor of financial services.

At a time when American business faces its toughest competitive battles at home and abroad, a growing number of companies have made pivotal decisions not to compete in areas that at one time were their main sources of revenue.

In many cases, these companies are abandoning core businesses and key product lines to go into new areas that are less exposed to the crosswinds of competition. Corporate birthrights and namesakes become history as they are sold to competitors or spun off into new companies. Many Americans are scarcely aware that well-known products and services once associated with a particular company have changed hands.

For the companies involved, these divestitures are restructurings that are nothing less than radical surgery. They are key decisions that come as a response to economic pressures, the advances of corporate raiders or the demands of shareholders for improved financial performance.

Most affected are companies that have fundamental competitive problems. Often there is a cancerous, unstoppable decline of their basic business. The prospect of major capital outlays to modernize, or high labor costs, or the effects of deregulation and tough foreign competition can force companies to become something quite different from what they historically have been.

"Many companies find there is nothing they can do as an individual firm that will fix their position, except getting out of the business," said Joseph Bower, a professor at Harvard Business School and author of a book called "When Markets Quake," an analysis of restructuring in the petrochemical industry.

For industries that are unhealthy, Bower said, the biggest problem often is global overcapacity -- such as the situation in steel and small cars.

So imposing are these challenges that even industrial giants find themselves concluding that they would rather switch than fight.

General Electric's recent decision to sell its $3 billion-a-year consumer electronics business to Thomson S.A., a French electronics company, grew from GE's belief that Far Eastern competition in areas such as televisions and radios might not be worth battling -- even for a company that played a pioneering role in the development of those products.

The GE move is also illustrative of a broader trend in American business of companies focusing more closely on a small group of strong performers that have global impact, instead of hanging on to widely diverse lines of businesses that do not sufficiently add to the bottom line.

In fact, divestiture has become a major part of the corporate regimen in the 1980s, just as diversification was a staple in the 1960s and 1970s.

As companies realize that they cannot manage businesses they acquired, that they have become sitting ducks for corporate raiders, or that foreign competition is pummeling them, they begin the often painful process of changing their spots.

For these three companies, a combination of long-term planning and looking to more lucrative markets led to these decisions:

If you associate a whiff of diesel or a flash of silver in the sunlight with the name Greyhound, it's gone forever, save a modest interest the company kept in driving buses.

Last March, after Greyhound Corp. Chairman John Teets had a final falling out with unionized employes in the bus line, the foundation of the corporation that was laid in 1914 by a Swedish immigrant who shuttled miners between towns in Minnesota was sold for $380 million to a private group of investors.

Teets was persuaded to sell by body blows such as airline deregulation, bus deregulation that put low-cost competitors on the road, and bitter labor problems that resulted in a 47-day strike against the company in 1983. "If you are sitting there with companies that are facing deregulation or foreign competition, there has to be a strategy for that. Digging in your heels isn't enough," said Teets. With or without the bus line, the strategy is for Greyhound to become what Teets calls a "consumer-service type company."

Though many consumers may not know it, Greyhound has been traveling down that road for some time. Dial soap, Ellio's frozen pizza, Brillo pads, the management of a group of hotels in Montana's Glacier National Park, a money order business, a cruise line and the production of intercity buses are some of Greyhound's markets.

If the name Singer means sewing machines to you, try instead exotic radar-threat warning systems, simulators that train fighter aircraft pilots and commercial aviators, power tools and residential gas meters.

Known as a sewing machine company since 1850, Singer last year completed its transition into an aerospace electronics company that now derives 80 percent of its revenue from high-technology operations.

The business of manufacturing sewing machines, which was started by in a Boston machine shop by Isaac Merritt Singer with $40 in borrowed capital, has been spun off into a company known as SSMC Inc., a slimmed-down sewing machine and furniture unit in which Singer holds a 15 percent interest.

American Can last year was transformed from the unglamorous business of producing food and beverage cans -- the business it was founded upon 85 years ago -- into Primerica, a financial services and specialty retailing company that recently purchased the Smith Barney, Harris Upham & Co. brokerage.

The strategy was laid out by William Woodside, who retired in January as chairman of American Can. Some 25 of the company's businesses were divested by the time of his retirement, and the ascendency of financial dealmaker Gerald Tsai Jr. in the company assured its movement away from capital-intensive, low-growth manufacturing into high-growth areas -- insurance, asset management, mortgage banking, direct mail marketing and specialty stores.

"There are occasions when the divestiture of a mature business {to} another mature business creates something that is exciting and growing," said Kenneth Yarnell, Primerica's president and chief operating officer. "We probably have done the country a favor."

But the question remains whether getting out of a business, instead of improving it, is good for the nation's overall competitive posture and its investment in manufacturing -- particularly if the new owners are foreign.

"I do have the concern that many managements have failed to search diligently and creatively for ways to reinvigorate old businesses," said Carl Sloane, president of Temple, Barker & Sloane, Boston-based management consultants.

TRW economist Pat Choate calls it a middle-aged identity crisis for many firms. "Is it a caterpillar becoming a butterfly or a butterfly becoming a caterpillar?" asked Choate, who attributes successful restructurings to "the quality of thinking of the chief executive officer."

One way to measure the success of a restructuring is how it is received on Wall Street. That's the short-term analysis.

Primerica's common stock, for example, shot up from a low of about $26 per share in 1982 to more than $105 per share before the stock split in March. Much of the change that has occurred in the chemical industry has been cheered by financial analysts.

But opinions on whether the restructuring craze will hurt or help America's competitive position now and in the long term are as mixed as the results companies have after they embark on such a program.

Steve Waters, codirector of Shearson Lehman Bros.'s mergers and acquisitions department, suggested that divesting major parts of businesses may, in the final analysis, turn out to be a wash.

"People who are not competing divest to other people who are willing to compete," said Waters.

Triangle Industries, for example, is strong in packaging and bought Primerica's can-making business for $600 million last year. This year, Triangle will pump some $250 million into building new plants and modernizing facilities, an investment it says will yield immediate results.

Another outcome of companies moving on to seemingly greener pastures is that concentration is hastened in many industries, or foreigners amass larger American market shares. Though there is significant competition in the packaging business, Triangle's American Can acquisition made it the world's largest packaging company.

GE's exit from domestic color television production leaves Zenith as the last major American producer to carry on the battle against foreign competitors.

But not all companies decide to raise the white flag in the face of of a competitive battle -- domestic or foreign -- and come out of the fight a winner. American Telephone & Telegraph is a case in point.

In the aftermath of the court-ordered breakup of AT&T, the business of selling telephones to residential customers fell to AT&T from the local operating companies.

Suddenly, the new consumer products division faced a hoard of low-cost competitors, consumers who began buying rather than leasing telephones and the overriding perception that a phone was a phone -- no matter who sold it.

"Everything that we were in January 1984 looked more like our past than our future," said Kenneth Bertaccini, AT&T vice president of consumer products.

AT&T was so shell-shocked that it thought the unthinkable: Should it get out of the then-unprofitable and tough business of marketing home phones? Instead, it launched what it called Project Turnaround, an aggressive, top-to-bottom overhaul of the business.

New faces were added to the management lineup, about half the product line was eliminated, leasing rates were guaranteed for two years, an ad program stressing quality was launched, residential phone manufacturing was moved offshore, many Phone Center stores were closed, and expenses were cut in half.

Despite the pain of an internal reorganization, the payoff has been big. The consumer products division is now generating profit margins that are among the best within AT&T. The unit has maintained market share in corded phones, become the market leader in cordless phones and doubled its market share in answering systems since 1984.

Other companies, which undergo restructurings that leave the heart of the company intact but add new limbs, also consider their strategies to be competitive responses to changing conditions.

Richard J. Mahoney, chairman of the Monsanto Co., has pushed his company through an extensive restructuring program, paring the company's original bulk commodity chemicals business to a fraction of what it once was while forging ahead on the frontiers of science -- biotechnology, pharmaceuticals and higher-value specialty chemicals.

"We set out to identify ourselves as one of the great industrial enterprises," said Mahoney. "I don't think it would have been possible {without the restructuring}. We would have been a second-tier company if we just let it ride."

The prescription for facing intense foreign competition and stagnating earnings was to buy a pharmaceutical company, G.D. Searle & Co., and its NutraSweet unit. Research and development dollars -- some 40 percent last year -- were shifted to "emerging technologies." Along the way, more than $4 billion in chemical and commodity related businesses were divested.

The strategy, Mahoney realizes, hinges on the company becoming a successful inventor and developer. "We have to turn a huge research and development investment into products -- fast. The biotechnology revolution has to happen," he said.

For better or for worse, the success of such strategies depend on the leadership of chief executive officers, the timing employed and the ability of the organization to change its corporate culture.

Some predict that going too far from what a company knows and used to do best can be courting disaster. "You sell your roots for a bag of beans," said Jack Reichert, chairman of Brunswick Corp., which has clung to one of its oldest businesses, bowling, despite the much diminished contribution it makes to earnings as the company has diversified into boats.

"It's bad that sometimes there is a tendency to walk away too soon from the things you are good at," agreed Thomas Hustad, a marketing professor at Indiana University's graduate school of business. "But there is a time to cut and go on to to something else."