(+/-)-4-Methylthioamphetamine (MTA) was resolved into its enantiomers, and a series of N-alkyl derivatives of the parent compound, as well as its alpha-ethyl analogue, were prepared. The monoamine oxidase (MAO) inhibitory properties of these substances were evaluated in vitro, using a crude rat brain mitochondrial suspension as the source of enzyme. All compounds produced a selective, reversible and concentration-related inhibition of MAO-A. (+)-MTA proved to be the most potent inhibitor studied, while all the other derivatives were less active than the parent compound, with (-)-MTA being about 18 times less potent than the (+) isomer. The analysis of structure-activity relationships indicates that the introduction of alkyl substituents on the amino group of MTA leads to a reduction in the potency of the derivatives as MAO-A inhibitors, an effect which increases with the size of the substituent.