A-Ethyl-a-methyl succinimide (PM 671) had a high and dramatic degree of effectiveness against pure petit mal epilepsy with a relatively low incidence of harmful side effects. An overall 85% reduction in attacks was obtained in the 109 patients studied. Complete control was secured in 42% of patients; practical control with an 80 to 99% reduction in attacks in 24%; and partial control, 5 to 79% reduction, in 26%. PM 671 is also outstanding in the lack of "slippage".