Despite a drop in the prime rate to 11 1/2 percent, one-third of the credit card issuers surveyed this month by the Maryland attorney general's office have increased their interest rates on finance charges above 18 percent.

Attorney General Stephen H. Sachs' office reported that 18 out of 53 banks and other financial institutions, department stores and oil companies raised their maximums following an increase on July 1 in the statutory ceiling to 24 percent. The highest rate -- charged by several department stores -- is 21.6 percent. The lowest is 12 percent.

Of the 29 banks and savings and loans responding, 23 raised rates; 13 out of 16 department stores; three out of five oil companies. This is the fourth study of credit card terms by the attorney general's office.

The accompanying chart gives data for department stores and oil companies operating in Maryland. A chart showing rates offered by banks and savings institutions was published Nov. 15 in Washington Business.

The annual percentage rate (APR) is the yearly interest rate on finance charges. Minimum monthly payment (MMP), where full payment is not required each month, is shown as the greater of two figures, a dollar amount or the percentage of the outstanding balance.

Float time indicates the number of days after the transaction before payment is due. If two figures are given, the second is the maximum time allowed when a purchase is made just after the first billing cycle closes. The method of computation is described under inclusive and exclusive. The first means the average daily balance inclusive of unpaid previous finance charges. The second means the average daily balance exclusive of unpaid previous finance charges.