Differential development of monoamine oxidase (MAO) isoenzymes in rat whole brain is described in postnatally developing Sprague-Dawley rats. Total MAO and isoenzyme activity was measured using nonspecific and specific substrates. Total MAO activity measured using tyramine, increased postnatally up to 24 weeks of age and attained a plateau afterward. The increase in total MAO activity was significant at all age groups (18 days to 36 months) investigated as compared to new born rats. MAO-A and MAO-B activities were measured using octopamine and benzylamine respectively. We also observed a marginal increase of MAO-A activity and a significant increase of MAO-B activity upon development. Furthermore, at 12 weeks of age, MAO-B activity increased by 10-fold as compared to new born and was consistent up to 36 months of age. The qualitative localization of the enzyme activity on non SDS-PAGE by nitroblue tetrazolium staining confirmed the increase of MAO-B during the development. It is suggested that the maturational increase of total MAO activity in brain is predominantly due to the increase of MAO-B isoenzyme.