Introduction of high-yielding insured money market accounts at banks and thrift institutions this week coincided with the greatest fund outflow ever suffered by money market mutual funds--a plunge of $4.73 billion.

So much money has gone into the new accounts, which started Tuesday but were heavily promoted in advance, that some institutions have begun to retreat on the high rates offered initially.

According to the Investment Company Institute, the trade association for mutual funds, fund assets dropped to $225.7 billion this week. Broker-dealer money funds decreased by $2.78 billion, institutional funds by $1.3 billion, and general-purpose funds -- those in which individuals usually invest -- by $673 million. Institutions and professionals are more sensitive to interest rate changes than individuals and tend to move their money faster.

The ouflow was 2 1/2 times the size of last week's $1.8 billion decline in assets. In addition to the promotional offers made on the new accounts, an ICI spokesman suggested other contributing causes for the huge outflow, ranging from seasonal factors -- money withdrawn for holiday shopping -- to changes in interest rates for other kinds of investments. Treasury bills were mixed at last Monday's auction, with three-month bills rising to 7.995 percent and six-month bills declining to 8.205 percent. The seven-day average of money funds calculated by Donoghue's Money Fund Report dropped yesterday to 8.26 percent from 8.34 percent.

Heavy advance promotion of the Market Deposit Account in the Atlanta area resulted in a $700 million flood in deposits, according to securities analyst John Moore, vice president of Robinson Humphrey American Express. Consequently, three banks there reduced their rates from more than 20 percent to between 10 and 11 percent to protect their financial integrity.

In the D.C. area, Providence Savings of Vienna was obliged to cut its promotional rate from 14.06 percent to 11.36 percent after an influx of $22 million in the first 2 1/2 days of the new account. Vice President Dick Tubbs said the S&L became concerned about its cost of the premiums, which will total $125,000 for the first three days. He said S&L officials are trying to find ways to invest the $22 million at high interest rates, but admitted that not even commercial paper would earn the needed 15 percent.

Tubbs said the average account amounted to $22,000, although there were several accounts opened in excess of $100,000. Providence found that 84 percent of the money it received came from outside sources. Of that, between 70 and 75 percent came from money market funds, according to Tubbs, who reported seeing checks written on the funds for as much as $95,000.

For years, depository institutions watched their dollars go to money market funds. Now the money markets fear the trend will be reversed as investors seek similar yields covered by federal insurance.

Not to be outdone, Dean Witter Reynolds this week became the first money market fund to announce it would offer the same insured account. Dean Witter can do this because it is owned by Sears, Roebuck, which also owns Allstate Savings and Loan Association in Glendale, Calif. Other money funds are rumored to be planning similar moves.