The Fortune 500 continues to look like the roster for The Dating Game.

PepsiCo Inc. wants to buy most of Seven-Up Co., in a combination of the second- and third-largest soft drink companies. B. F. Goodrich Co. and Uniroyal Inc. plan a joint venture that would be the domestic tire industry's second-largest producer. General Electric Co. and RCA Corp. get engaged.

The nation's largest bank holding companies bid for smaller crippled banks and thrift institutions.

And in the oil industry, which already has produced a handful of billion-dollar mergers in the 1980s, some of the big oil companies are expected to be among the most active buyers for smaller parts of the industry that have become casualties to falling oil prices.

The parade of deals involving the nation's largest firms appears more pronounced than at any other time in the postwar period.

Deciding whether this trend is healthy or harmful for society and the economy is the principal responsibility of antitrust regulators at the Justice Department and the Federal Trade Commission.

Over the past century, antitrust regulation has produced a tug of war between two poles -- the populists on the left, with their ingrained hostility towards big business, and the Social Darwinists on the right, with their conservative belief in a competitive survival of the fittest.

Under the Reagan administration, the needle has swung over to the right. Regulators are taking a more permissive attitude toward mergers, appearing more inclined to find economic benefits from mergers rather than economic risks.

But the essential issue the regulators face has remained the same: Is a merger of two companies likely to produce a stronger competitor, creating a better marketplace for consumers in the process?

Or does a merger significantly reduce the number of effective competitors in a market, making it more likely that the remaining companies will figure out a way to boost prices above competitive levels, either through direct collusion or by conscious imitation of each other's pricing decisions?

The proposed Pepsi/Seven-Up merger and the Goodrich-Uniroyal venture provide good tests of how these old questions will be tackled by the Reagan administration as it seeks to make a lasting mark on antitrust policy.

Pepsi's intention to purchase most of Seven-Up is "one of the most seminal events in the industry's history," according to analyst Jesse Meyers, publisher of the Beverage Digest newsletter.

On the plus side, it appears to tighten the competition even further between Coca-Cola, the industry leader, and Pepsi, at No. 2.

Coke's various brands give it a 39 percent share of the soft-drink industry. Pepsi now has an estimated share of 28 percent. Adding Seven-Up would boost Pepsi's share to 35 percent. It also would give Pepsi an important new product in the fast-rising market for lemon-lime drinks.

On the other hand, the merger would have the opposite impact on the smaller competitors in the soft drink market -- Dr Pepper, RC Cola and Sunkist. This is a business in which volume is crucial, Meyers noted, and the combination of Pepsi and Seven-Up would put the others at an even greater disadvantage.

"These are two sizable entities that are merging with each other," said Lawrence J. White, a business professor at New York University and former chief economist at the Justice Department's antitrust division. "The risks of noncompetitive behavior are heightened . . . and I just don't see an overwhelming story on the efficiencies to be gained" by the merger.

The proposed venture between Goodrich, the third-ranking U.S. tire manufacturer and Uniroyal, the fifth-ranked, has not excited much concern on antitrust grounds thus far.

That's because they are competitors in different segments of the tire market, and each company's strengths appear to complement the other's, rather than multiplying their power in a single market, analysts said.

Uniroyal is one of the major suppliers of original tires for new U.S.-built cars, but Goodrich gave up on that market several years ago. Goodrich and Uniroyal build passenger car tires for the replacement market, but neither is a dominant player in that extremely competitive battleground.

When regulators look at mergers that reduce the number of effective competitors to a handful, they worry not just about open collusion, White notes. There is also the risk that the remaining firms may fall in step, copying each other's pricing or other strategic decisions while easing up competitively, to share higher profits.

That strategy won't work if the market remains competitive. It will be too easy for smaller rivals to enter the fray with cheaper products or services, taking business away from the collaborators.

That kind of low-cost, cut-throat competition from foreign manufacturers and powerful retailers characterizes the replacement-tire business.

The soft-drink market doesn't have that competitive shape. But the blood rivalry between Coke and Pepsi doesn't seem to create much of an atmosphere for cozy cooperation, despite their dominance of the market.

It is factors like these -- the ability of new firms to establish solid beachheads, and the size, strength and competitive instincts of the largest firms -- that should continue to guide regulators' decisions on mergers, White and many other antitrust experts insist. They are watching to see how the Reagan administration applies these old standards in the future.