The slump in downtown office rental activity that has resulted in unprecented vacancy rates is no laughing matter where developers, leasing agents and lenders are concerned.

So it seemed rather incongruous when some participants in a recent commercial real estate seminar resorted to jokes about the plight of developers who are stuck with nearly empty buildings.

Perhaps the gallows humor was intended to connote that misery loves company. Or perhaps the series of one-liners about the plight of certain property owners and lenders were really insider jokes. At any rate, they didn't go over very well with some who attended the session last week at the National Home Center.

In fact, there were those who considered the pointed and comedic references to some slow-moving (high-vacancy) projects as more than a bit tacky.

Depending on which expert's estimates you believe, 4 to 5 million square feet of office space will be available for occupancy this year in the District. Add the projections for the suburbs, and the total office space expected to be available in 1983 is approximately 7 million square feet.

And when developers and lending partners are stuck with projects that are 65 to 70 percent unleased, it's hardly a laughing matter. Optimistic forecasts notwithstanding, the outlook for substantially reducing vacancy rates doesn't appear to be very bright for the remainder of this year, at least.

The oversupply of space puts Washington's vacancy rate above the national average, and it probably will be next year before prospective tenants absorb the current supply, most experts predict.

On the other hand, last week's seminar, which was sponsored by the Commercial Industrial Investment Council of Washington, provided some heartening forecasts for commercial property owners and leasing agents. It wasn't all fun and games.

For the short term, at least, the outlook for reducing the space glut is promising from the perspective of two of the panelists who participated in the forum. The key, both emphasized, is the relative stability of interest rates.

Developers have been accused of poor planning in miscalculating demand for office space in the recent construction binge, mostly east of 15th Street NW. But in all fairness, few experts had predicted the course the economy would take. High interest rates and a general downturn in the economy forced potential office users to delay or cancel plans, and the boom that had been anticipated in the office market fizzled.

But the recent decline in short-term interest rates signals a "good omen for the next few months ahead," observed Daniel J. Callahan III, vice chairman at American Security Corp.

Although interest rates are likely to decline for a while, Callahan added, "long-term, I think you have to be worried."

Echoing Callahan's recitation of the interest-rate scenario, real estate executive James L. Eichberg projected a substantial increase in office rental activity--"as long as interest rates remain stable."

In a conversation later, Eichberg, who is president of Smithy-Braedon Co., spoke buoyantly of "a very active market" that has developed within the past two months. "I feel the market has hit bottom and people are making deals," Eichberg said.

Earlier, Eichberg assured industry representatives that "commitments are being made and those prospects previously in the market who dropped out because of the economic uncertainties of our nation are now returning with a much brighter outlook for expansion."

That's a plus for the local economy as well as good news for owners of the new generation of office buildings that have been hit by a space glut.

By most accounts, expansion in the downtown and suburban office markets will continue to be supported by the three A's--associations, attorneys and accountants. But demand--assuming continued improvement in the economy--will be generated primarily by international trade and banking and the high technology and telecommunications industries, commercial real estate interests are being told.

If those forecasts hold up, a good many in the office market industry who got burned last time around will find a lot to laugh about.