According to Eysenck's (1957) drug postulate a stimulant drug should decrease cortical inhibition, increase cortical excitation and thereby produce introvertedness; whereas, a depressant drug should decrease cortical excicacion, increase cortical inhibition and thereby produce extravertedness. In a recent experiment, Costello and Eysenck ( 1961 ) found that extraverrs showed greater persistence on a physical task than introverts, a finding consistent with Eysenck's ( 1957, 1960a) general theory of personality. Therefore, in the present experiment a stimulant drug was expecced to decrease, and a depressant drug, to increase S's persistence on a physical cask. A group of 30 male smdents between 17 and 20 yr. served as Ss, their mean weight being 115 Ib. (SD = 9.03). First, each S was given 6 trials of strength, 3 trials with each hand, on a hand dynamometer with 30-sec. intervals berween trials given in the R L R L R L order. From the scores two-thirds of mean strength shown on six trials by 5 was calculated. Afterwards, following an incomplete Latin square design 100 mgm. of phenobarbitone, 7.5 mgm. of Dexedrine, and a placebo (peppermint) were administered to each S on three consecutive days. The drugs and placebo were given orally, and in identical tablets. One hour after the administration of the drug each S was given mro trials of persistence, one with the right hand and the other with the left hand, at his twothirds setting on the dynamometer. S was instructed to keep rhe pointer as long as possible at that setting. The duration of persistence was timed with a stop watch. Each S's mean persistence score for two trials was calculated, and analysis of variance was performed on che data. Only the variance due to drug effects was found significant, che level of significance being ,001. The mean persistence scores for phenobarbitone, placebo, and Dexedrine were 22.43, 17.01, and 13.80, respectively, and were in the predicted order. Application of a t test to the differences becween the various treacmenc mean scores disclosed highly significant differences between the phenobarbitone and placebo treatments (p = ,021 and also becween phenobarbitone and Dexedrine treatments (p = .01). The difference between the placebo and Dexedrine treatment means did not reach a statistically acceptable level of significance. Thus the depressanc drug (phenobarbitone) significantly increased, while the excitant drug (Dexedrine) decreased, though not significantly, persistence measures on a physical task. The results substantiated the prediction.