The inhibitory activity of methylphenydate and of threo- and erythromethylphenydate in homogenates from liver, brain and kidney were investigated. This inhibition was also demonstrated in vivo. The monoamine oxidase inhibition produced is of similar strength in these two cases. The acute hypertensive actions and toxicities of the threo and erythro isomers are of equal strength; but, in agreement with other work, only the threo derivatives display a locomotor stimulant action. No parallelism could be traced between the locomotor stimulant effect of stereoisomers and their inhibition of monoamine oxidase.