An automated exchange for the sale of large denomination certificates of deposit to institutions is scheduled to begin operations this week in Washington.

Called CDx, the computerized service will enable regional and local banks and savings and loans around the country to compete effectively against big money center banks in marketing their CDs to pension and money market funds, trusts, insurance companies and other investors on a same-day basis. About 800 financial institutions have signed up.

Until now, smaller banks have sought buyers for their CDs either directly through newspaper advertisements or by participating in nonautomated pools set up by big brokerage firms, which do not offer instant execution of orders and same-day settlement. One consequence has been the growing concentration of funds in a few large institutions resulting in a flight of capital from many parts of the country.

CDx offers transactions at up-to-the-minute market rates. The system, which is carried over telephone wires, was developed by Harvey Basking & Co., a financial services corporation located in the Flour Mill in Georgetown. Transactions are simultaneously received at the New York office of State Street Bank and Trust Co. of Boston, which acts as a clearing house. It transmits proceeds to the issuer through the Federal Reserve System and issues bearer certificates to the purchaser.

In the Penn Square fiasco, a number of credit unions and savings and loans were burned by buying uninsured obligations offering premium rates. CDx deals only in $100,000 CDs insured by the Federal Deposit Insurance Corp. and the Federal Savings and Loan Insurance Corp.

In order to guarantee that the entire purchase of $1 million or more is insured, CDx's computer will not allow an institution to buy a package containing a CD from any issuer whose paper is already in its portfolio. The computer can also select CDs from a particular part of the country or from issuers of a certain asset size, if the institution wishes.

Interest rates and maturities -- ranging from 14 to 360 days -- are set by the issuing bank or S&L. While the rates may vary from customer to customer -- due to the necessity of weeding out duplicate CDs from the same issuer--Baskin says the average fully insured package is expected to yield between 20 and 25 basis points more than largely uninsured packages issued by money center banks. (A basis point is a hundredth of a percentage point.)

The cost of the service is $47 plus one basis point per maturity month. Thus charges for selling a 30-day CD would be $57, or $47 for the transaction plus $10 basis-point fee.

Oakley Hunter, who retired recently as chairman of the Federal National Mortgage Association, is chairman of the CDx advisory committee. Other board members include former Federal Reserve governor Philip C. Jackson; former Federal Home Loan Bank Board chairman Robert H. McKinney; James O'Leary, former vice chairman of U.S. Trust Co. in New York, and Jay Kislak, a Miami mortgage banker.