Tomorrow marks the official debut of the Money Market Deposit Account -- the banks and savings institutions' challenge to the money market mutual fund.

Like millions of Americans elsewhere, people in the Washington area will at last be able to earn a market rate of interest on their federally insured deposits.

The new account, mandated by Congress and designed by federal regulators, requires an initial deposit of $2,500. Financial institutions are allowed to guarantee a given rate for as long as one month but may also change it daily. If the average monthly balance falls below that amount, the entire account will earn interest at the rate of 5 1/4 percent.

Regulations limit account holders to six transactions per month, although that should not be taken literally due to the exceptions.

The restriction applies only to telephone transfers from one account to another in the same institution and checks written against the account. The maximum number of checks permitted is three, plus three telephone transfers. Or if no checks are written, six telephone transfers are allowed. But an unlimited number of withdrawals and deposits may be made at any time either in person, by messenger or by mail.

Financial institutions must monitor the number of transactions to be sure regulations are not violated and notify customers who do not comply. But they are also free to add deterrents in the form of fees.

The account does not have a fixed maturity, so there is no penalty for early withdrawal. Nevertheless, institutions may levy penalties if they wish. Similarly, there are no regulations governing the size of deposits or withdrawals, fees charged for transactions and maintenance. Again, it is up to the institution to set its own terms.

This account should not be confused with another new account, the Super NOW, which becomes effective Jan. 5. The Super NOW (for Negotiable Order of Withdrawal) will have many of the same characteristics, with its principal distinction being the unlimited number of checks that can be written on it. If the transaction restrictions on the Money Market Deposit Account seem too stiff or confusing, customers may wish to wait three weeks and open a Super NOW account. The rate on this is expected to be approximately one percentage point lower than on the Money Market Deposit Account.

Although a few banks and savings and loans have tried to sign up customers in advance with offers of high rates payable before Dec. 14, most institutions are officially launching their campaigns effective tomorrow.

A random survey made late last week by The Washington Post of 22 federally insured banks and savings and loans in the District, Maryland and Virginia reveals a wide variance in terms. Whereas they are almost unanimous in offering the account at the minimum level, $2,500, they promise to pay interest at rates varying widely from 7.96 to 20 percent.

The highest rate, 20 percent for 17 days is promoted by Suburban Bank in Maryland, but it is also the only bank of those queried to set an initial deposit of $5,000, twice the required minimum. Anyone opening an account would earn $47.22 in simple interest in December.

Compared with savings and loans, banks showed a wider variance in rates. The difference between the highest and lowest is 12.04 percentage points. On average, their initial rate equals 11.21 percent, while S&Ls are paying an average of 10.7 percent. The difference between 13 percent, the highest rate offered by an S&L, and the lowest is just 4 percentage points. Standard Federal Savings and Loan of Gaithersburg guarantees 13 percent for a month.

It should be noted that half of the interest rates listed carry time limits and hence should be considered promotional rates that will probably decline after the expiration date to nonpromotional rates. However, since 40 percent of the institutions surveyed had not decided on their rates by last Friday or were unwilling to divulge them for competitive reasons, it is premature to draw conclusions about typical rates in this area. Of those responding, the average of the "regular" rates is 8.36 percent, a figure more in line with short-term interest rates paid by money market mutual funds.

Some institutions peg their rates to public statistics. The latest figures available for two of the most widely followed indices of short-term rates, the 91-day Treasury bill rate and Donoghue's Money Market Fund Average, are 7.96 and 8.3 percent respectively.

Yet two thirds of the institutions in the survey replied they did not plan to use any external index. While most regarded their formulas as proprietary and would not reveal how the calculations are made, a few said they would base yields on a combination of government indices. Others admitted they would look at their competition.

Nearly three quarters of the institutions place no minimum on the size check a customer can write against the account. In the few cases where minimuns are set, the most popular figure appears to be $500, the same minimum imposed by many money market funds. Two banks disallow checks entirely.

Slightly over half of the institutions queried levy fees on some aspect of the account: transaction fees per check, deposit or withdrawal, often above a specified number; or so-called monthly maintenance fees or management fees. The penalties for exceeding the number of transactions allowed by law or for allowing the balance to fall below $2,500 are often onerous.

American Security Bank charges the highest monthly maintenance fee, $6; and Dominion Federal S&L, the stiffest penalty for excess transactions, $15 each.

First Women's Bank of Maryland will deduct $50 from the balance if the account is closed within 90 days after it is established. Twice as many banks as S&Ls in the survey chose to impose fees on transactions.

Only one bank questioned, Riggs, is offering a cash bonus program to open an account. It will pay a low of $10 for a new account with a minimum deposit, up to a high of $80 for a deposit of $20,000 or more. First Women's Bank of Maryland will give away merchandise premiums in conjunction with its new office opening.

Finally, First Women's is the only bank in the survey to propose two different types of Money Market Deposit Accounts for individuals. (Others require a higher initial balance from businesses.)

For the person with $10,000 to deposit and wanting other banking services, the bank will allow three wire transfers a month in or out of the account, a 25 percent discount on a safety deposit box rental, one year's supply of free checks, and a discount on handling fees on the purchase of Treasury securities. Suburban Bank has also chosen to package the new account with other services. It will waive the first year's fee of $48 for a Visa Gold Card, and allow a 1 percent discount on auto loan rates.