The effects of magnesium pemoline were compared to those of caffeine and methylphenidate on the performance of a nonmotivated task which required continuous attention over a two hour testing period. The drug treatment consisted of: 25 or 50 mg magnesium pemoline, 100 or 200 mg caffeine, 15 mg methylphenidate or a placebo. Ten unhospitalized volunteers were each tested six times, once on each treatment. The significant increase in errors which occured under placebo conditions did not occur with 50 mg magnesium pemoline, or 200 mg caffeine or 15 mg methylphenidate. These results support the contention of others that magnesium pemoline acts as a nonspecific CNS stimulant.