The present work is an attempt to investigate the Drug Postulate of Eysenck in the animal field. The Ss were 36 male Wistar rats. The extraversion measure used was animal ambulation in a low-frightening open-field. d-Amphetamine, reserpine and saline were administered before the extinction of a lever-press FR 25 schedule for food. The results show that reserpine produces a decrement in the resistance to extinction (RExt), while d-amphetamine produces a biphasic effect, increasing the RExt in high-ambulatory (extravert) rats and leaving the RExt of low-ambulatory (introvert) rats unaffected. Therefore, reserpine acted as a depressant drug, d-amphetamine as a stimulant drug and ambulation as extraversion. © 1985.