In one corner of the run-down red brick building, a split bowling ball bolted to a metal rod pile-drives onto a twin-size mattress again and again, testing it for firmness and resilience.

In a quieter spot not far away, several staff members who look like white-coated doctors are baking chocolate chip cookies from an assortment of recipes; a squad of testers nearby eat the finished products.

Meanwhile, in another area, packed suitcases are spun around in a metal drum that subjects them to abuse even a gorilla baggage handler might have trouble contriving.

After struggling through two years of poor financial results by its own rating, Consumers Union, the world's largest independent product-testing organization, is alive and kicking.

Kicking doors, that is, poking mattresses, charging car batteries, dropping telephones, tasting cookies and climbing ladders.

The perennial tester of everything from peanuts to paint, whose ratings are published in its monthly Consumer Reports magazine, has bounced back by streamlining its operations, trimming the magazine and receiving millions of dollars in contributions from its subscribers.

"Our financial situation has improved tremendously," said Rhoda H. Karpatkin, the executive director of the 49-year-old organization. "We were especially hard hit by the recession, because when consumers can't afford to buy products, the last thing they want to buy is a consumer-product magazine."

"Those days are hopefully behind us," said R. David Pittle, technical director of the not-for-profit organization.

A walk through CU's laboratories 20 minutes north of Manhattan in Mount Vernon, N.Y., illustrates the breadth of the organization's testing.

Throughout a maze of corridors and stairways, professional testers are washing dishes dirtied with stew and spaghetti, listening to blaring stereos, working out on rowing machines and switching electric hair blow-dryers on and off. Everywhere there are chain saws, washing machines, television sets and bottles of shampoo.

"We're extremely pleased that we're now on a very firm course," Karpatkin said. "Our cost-cutting measures helped us get through a very rough period, and we now have more subscribers to Consumer Reports than ever before." There are now 3.2 million subscribers.

Consumers Union's difficulties began in December 1981. The recession hit, precipitating a sharp drop in Consumer Reports magazine and book subscriptions, its main source of revenue. Then, two days before Christmas, the U.S. Postal Service nearly doubled the postal rates for nonprofit groups from 15.8 cents a pound to 27 cents a pound.

"The impact of the jump in postal rates was around $2 million in costs that we were not prepared for," Karpatkin said. CU lost $2.35 million in fiscal 1981.

CU's 243-member staff was reduced by one-eighth, the number of pages in Consumer Reports was reduced, some of its features were dropped and a cheaper type of magazine paper was used. The organization then switched from second to third class mail. But, still it struggled. In fiscal year 1982, CU lost $2.8 million, boosting its cumulative debt to $8.5 million.

"We thought we would have to close our only three advocacy offices in Washington, California and Texas," Karpatkin said. The advocacy offices have represented consumers in government agencies and Congress over the past decade.

So CU turned to its readers and asked for contributions. More than $3 million poured in. "We had a strong base of support among our own subscribers to help us out of the crisis," Karpatkin said. "They were like a family."

By 1983, CU's misfortunes had turned. In the 1983 fiscal year, the organization's surplus (its income minus expenses) was $1.4 million on revenue of $36.3 million. In fiscal 1984, the surplus was $4.4 million with $39 million in revenue. Its debt is almost paid off.

"It was one heck of a turnaround," said CU spokesman David Berliner.

But, the testers' troubles were not completely over. It was hit with a 13-week strike in January 1984 by 139 union members seeking a stronger voice in management.

The strike embarrassed management at CU, which was founded in 1936 by former union members of another consumer magazine, Consumer Research, who had split away from management. Last April, the CU strike was settled.

Most of CU's testing is done by its in-house experts. The testers include chemists, electrical enginners, chemical engineers, mechanical engineers, food and textile technologists, statisticians, biologists, home economists, market analysts and nutritionists.

In some cases, CU, with 210 of its own staffers, has to subcontract experts, such as "very expensive" taste testers who have been trained to distinguish between smooth, gritty, sweet and sour flavors.

CU's magazine pages, cut back during tougher times, have been restored, and the organization now tests more expensive and complicated products. The testers have a separate auto-testing facility in Connecticut and rent land in Florida for their annual lawn mower tests.

Because its goal is to offer objective advice to consumers on everything from lawnmowers to toothbrushes, CU doesn't accept any advertising for fear that it could appear to taint the published test results.

In addition, CU discourages manufacturers from taking advantage of CU ratings because it fears its credibility may suffer. So the magazine files lawsuits against companies who cite CU results in their national product ads.

Some critics say that CU should devote its limited resources to major household appliances and cars rather than testing goods whose rating depends largely on consumers' tastes, such as chocolate chip cookies and movies.

But Karpatkin says its choice of products to test reflects the broad interests of its subscribers, and its audience apparently is content enough with its choices. A 1982 Louis Harris poll showed people trusted Consumer Reports more than any other consumer service, including the Better Business Bureau.

CU does refuse on principle to test certain items. Guns, hard liquor and cigarette ratings don't appear in the magazine. "Certain products are inherently hazardous, and we wouldn't want to help to promote their use," Pittle said. "I don't think there's any way that we could rationally take a position that one cigarette is better than another."

The future looks bright for the testers, Karpatkin said. CU is raising funds to move from its antiquated building, constructed in the late 1800s, to a modern one nearby. It also plans to launch a children's consumer television show based on its children's consumer magazine, Penny Power. (To keep its tax-exempt status, CU must continue to invest its surpluses to further the organization's purposes).

Although CU is a bible to millions of consumers, it is not without its critics. Some companies, who perform their own tests, disagree with CU's results.

In October 1982, CU reviewed the Pritikin diet and concluded it was potentially dangerous because it was nutritionally inadequate and didn't live up to its claims.

"The Consumer Reports article was the most inaccurate portrayal of me and my program that I have seen in 20 years," said Nathan Pritikin, the founder and creator of the Pritikin Program for Diet and Exercise. ". . . It refused to print my documented point-by-point rebuttal or any of the numerous letters that protested the article from physicians and laymen familiar with the results of my program," he said. CU said that after an extensive review, it stands by its report.

In a few cases, its ratings have had a dramatic impact on companies' fortunes. Kero-Sun declared bankruptcy after Consumer Reports said its kerosene heaters gave off hazardous fumes. Bose Corp. was so outraged by a Consumer Reports rating of its stereo loudspeakers that it pursued a lawsuit unsuccessfully all the way to the Supreme Court.

CU is not antibusiness, Pittle said. "We don't have any axes to grind. We don't think business is something people need to be protected from. Our role is to take uncertainly out of the marketplace and enhance the ability of the consumer to be an intelligent and rational buyer."