1. The chewing of khat leaves as a stimulant is common in certain countries, and the effects of this material are supposed to be due to the phenylalkylamine alkaloid cathinone. In order to determine the effects of this substance in humans, a single oral dose of cathinone or placebo was administered to six healthy male volunteers in a double-blind, random order crossover study. 2. Cathinone produced increases in blood pressure and in heart rate, and these changes were concomitant with the presence of cathinone in blood plasma. 3. The physical and mental changes that the subjects reported during the experiment indicated that cathinone has in humans euphorigenic and psychostimulant effects. 4. These observations support the assumption that cathinone is the constituent mainly responsible for the effects of khat, and they show that this alkaloid has also in humans amphetamine-like effects.