One of the prime products of banking deregulation, the money market deposit account, observes its sixth anniversary this month.

But many savers and investors might feel they have less reason to cheer the occasion than the banks and savings institutions that take these deposits.As short-term interest rates have climbed steadily this year, yields on most types of money market investments have moved up on a parallel course.

In contrast, returns on the typical money market deposit account, or MMDA, simply haven't kept pace.

As of late November the average rate on MMDAs at 100 large banks and thrift institutions stood at 5.93 percent, according to Bank Rate Monitor.

Compare that to a 10 percent to 10.5 percent prime lending rate, the basic rate used by the institutions to calculate the interest they charge on many adjustable-rate loans to both businesses and consumers.

Or, even more pertinent, contrast it to the average yield on money market mutual funds of just under 8 percent, as reported by Donoghue's Money Fund Report of Holliston, Mass. That translates into $4 in interest for every $3 paid to MMDA depositors.

MMDAs were created in December 1982 as a way of allowing banks and savings institutions to compete with the money funds, which had been enjoying explosive growth.

And compete they did. For most of the next four years, MMDAs offered returns close to - even occasionally exceeding - what you could get in a money fund.

What's more, MMDAs offered the advantage of federal deposit insurance. Money funds, no matter how safe they may be deemed, have no such direct backing from the federal government. Cash poured into MMDAs.

Since early 1987, another key difference between the two vehicles has begun to manifest itself.

Money funds are required to pay out to their shareholders whatever they can earn from money-market securities such as Treasury bills and commercial paper issued by corporations, minus their expenses and management fees.

Bank and thrift institutions, on the other hand, are free to pay whatever rate on MMDAs they think the market will bear.

Since interest rates began moving higher in the spring of last year, money fund yields have responded automatically with increases of their own. MMDA yields barely budged.

The disparity in available returns has not gone unnoticed. Assets in money funds have lately climbed to new highs, surpassing $280 billion.

That total still pales, however, in comparison to the $500 billion-plus that reposes in MMDAs.

"This indicates that banks and thrifts have found a level of interest rates satisfactory to most depositors that is significantly below the rates prevailing in the open market," said Norman Fosback in the advisory letter Income & Safety.

"By permitting their yields to lag far behind money funds, banks and thrifts have lowered their interest costs - and boosted their earnings - by approximately $7.5 billion a year.

"That same $7.5 billion represents earnings lost by savers that have chosen to ignore the growing superiority of money funds over deposit accounts.

"Of course, all insured money market accounts are not equal," Fosback added. In spots across the country, some institutions are offering MMDAs with rates in the neighborhood of 8 percent.

"Nevertheless," Fosback concluded, "the average money fund holder is doing much better today than the average money market account depositor at a bank or thrift."