How long the market's robust behavior will last is anybody's guess. But it's clear that numerous pension funds -- partly spurred by a firm push from corporate treasurers anxious for a better rate of return -- are fattening their equity holdings. And they are the country's biggest buyers of common stocks.

And still to be reckoned with, should interest rates give any hint of entering a downward trend, are the public's nearly $50 billion worth of those high-yield money-market funds. A healthy chunk of these funds could -- and probably would -- be committed quickly to equities.

Now some experts believe the market's zing could last another few weeks or so before a setback. And they're convinced that any setback will be minor.

It's tempting to get in, isn't it?

Well, for some thoughts on how an investor might play the current market, assuming he's willing to accept the ever-present and substantial risks, I focused on three concepts uppermost in the minds of many pros. They're undervalued situations (which everybody is always looking for), the always dangerous corporate takeover game and defense stocks (the market's current rage).

First to the undervalued stock and one analyst who heavily focuses on such securities: David Atkinson of Morgan Stanley & Co., which boasts one of the best -- and most expensive -- research staffs on Wall Street. Earnings momentum, dividends and return on equity are three of the chief things Atkinson looks at in trying to ferret out deeply undervalued situations. He also stresses the importance of focusing on companies that can be viewed in a favorable conceptual light (such as legalized gambling in '78, energy in '79 and, at present, defense).

Morgan Stanley recently wrapped up a list of 11 favorite undervalued securities. Among the group, Atkinson mentions three which make a lot of sense to the brokerage firm from the standpoint of value, earnings growth and conceptual ownership. Food processor CPC Iinternational, for example, is favored on two counts: the benefits it will derive from its participation in gasohol and the pressures on real personal income that will drive consumers to more basic food items. Northwest Industries (through its Lone Star Steel division) is seen benefiting as an important supplier of tubular products to the oil-drilling industry. And Atkinson says a favorable case can be made for Trane on the grounds that new air conditioners are much more energy-efficient than those of the past.

Rounding out the 11 are Moore McCormack, American Standard, FMC, Fluor, Owens-Corning, Timken, Stauffer Chemical and Macy's.

It should be duly noted that a number of these securities already have had big jumps this year. It's hardly a secret that the biggest bucks in the market are being made (all too often with inside information) in merger and acquisition deals.

One of the Street's hottest merger pickers is Igo Teichberg, the institutional research chief of Gruntal & Co.

Here's a list of his takeover favorites, a number of which already have been bid for and are up sharply: Barber Oil, General American Oil, Walter Heller, Harnischfeger, Integon, Kaiser Steel, Mohawk Data Sciences, Electrographic, GIC Investment, Texas International, and Joseph Dixon Crucible.

Last but not least is the hot defense group. Since last November, 20 leading defense stocks, fired up by increased U.S.-Soviet tensions, have risen an average 51 percent; in the same period, Standard & Poor's 500-stock index has advanced 9.2 percent. Clearly the investor is on a defense kick.

Should he continue to be in view of the sharp run-up?

Yes, says First Albany Corp. President George McNamee.

Back in November, just before the surge in defense issues, First Albany recommended three high-tech defense stocks: Loral, Edo and Rockwell International. As this column was being written, the trio had turned in a combined gain of just over 55 percent since November. Despite the huge advances, McNamee, who doesn't come across as a tout, expects all three to double in price from current levels within the next three years.

His reasoning is that there's no way that the United States can catch up quickly with the Russians in tactical aircraft and naval forces. Accordingly, he thinks the United States' only out is to focus on electronic countermeasures, namely adding highly sophisticated devices to existing defense systems.