If you're thinking about getting a new-home mortgage or refinancing an old one, you're face to face with one of the critical money quandaries of the coming four months: Where are interest rates headed in early 1986?

Is it true that the gentle downward slope of the mortgage-rate curve in 1985 has hit rock bottom and can go nowhere but up? Does it make sense to rush out and grab a 9 percent adjustable or an 11 1/2 percent fixed-payment loan this December, rather than run the rate-uncertainty risk of the new year?

Pose those questions to a distinguished panel of mortgage market and Wall Street economists and you could be surprised at the unanimity you'll find. Six out of six had the same answer: Interest rates could ease by still another small fraction between now and next spring, but don't miss a good purchase or sale opportunity waiting for that one-half percent drop to materialize.

The safest interest-rate assumption for anyone involved in real estate "is for relative stability -- no dramatic moves in one direction or the other" for much of the coming year, said former Federal Reserve Board member Lyle Gramley. The short-term outlook is for no more than a one-half percent decline during the next 90 days and an upward rebound of about the same magnitude by early spring, he said.

Gramley, who was a governor of the monetary-policy-setting Fed for five years until resigning this past fall, is the chief economist for the Mortgage Bankers Association of America. His insider's view is valuable in gauging the Fed's future moves to raise rates (by pinching the nation's money supply) or lowering rates (by easing monetary purse strings). In Gramley's view, at present, the Fed sees an economy that's growing slower than expected, "but that has plenty of vigor" and inflationary potential.

If the Fed determines that the gross national product is expanding at only 2 1/2 to 3 percent by the end Don't miss a good purchase or sale opportunity waiting for that one-half percent drop to materialize. of 1985, the board may loosen its monetary policy slightly, allowing for additional, minor decreases in the cost of money by mid-winter, Gramley said.

He added that the longer-term outlook for rates is more ominous unless Congress and the White House come up with a "truly credible deficit-reduction program" by early in 1986. In Gramley's view, a credible formula means a commitment to a federal deficit level of 1 1/2 to 2 percent of GNP by the end of the decade, but not necessarily a strict-regimen, fully balanced federal budget by then. Without such deficit controls, rates have nowhere to go but up from late 1986 on, he predicted.

Fellow mortgage-market pundits have independent analyses that come to the same bottom-line conclusions: Early 1986 is likely to be better than later. And 1987 through 1988 could well be worse than at present, absent dramatic congressional and presidential action on spending.

James Christian, chief economist for the U.S. League of Savings Institutions, thinks all this adds up to an "excellent opportunity" for buyers and refinancers to lock in rates now through 15-year mortgages. The economic and political odds weigh against any sharp breakthroughs that will send rates plummeting soon, he reasoned, making discount-rate, 15-year, fixed-payment loans "the smartest bet" in the marketplace now. The 15-year mortgages typically carry one-half percent lower rates than 30-year loans.

Federal National Mortgage Association Chief Economist Timothy Howard likes 15-year loans for the same reasons, but noted that their higher monthly payments rule them out for borrowers with modest cash resources. For such buyers and refinancers, rate-capped adjustables in the 9 and 10 percent range continue to be a safe, low-cost alternative for at least the next three years, Howard said.

Yet Howard conceded that adjustables entail a nagging risk that 11 percent, l5-yearmortgages or 11 1/2 percent, 30-year loans do not: If the deficit isn't brought under credible control in the next 36 months, and Gramley's worst-cast rate scenario occurs, borrowers with new adjustables could be starting at 15 1/2 percent rates in a few years.

What does former Fed member Gramley advise?" All I can tell you," he said with a smile, "is that I'm sticking with my fixed-rate mortgage" -- a 6 percent antique that will give him peace of mind into the next decade. .