Changes in biogenic amine content in the substantia nigra and in both forms of monoamine oxidase in substantia nigra and striatum of the rat during postnatal development (15-180 days) have been studied. Dopamine and serotonin had the same levels at day 15, however, each monoamine showed a different developmental profile. Dopamine levels and their metabolites (except 3-methoxytyramine) decreased during postnatal development. Serotonin levels and their main metabolite, 5-hydroxyindolacetic acid, underwent an increase during all stages studied. There were no statistically significant changes in noradrenaline levels until day 180 when they increased with respect to day 15. The highest activity of the monoamine oxidase-A in substantia nigra coincided with the highest 5-hydroxyindolacetic acid:serotonin ratio. Monoamine oxidase-A in the striatum did not change contrary to that which happened in substantia nigra. The monoamine oxidase-B:monoamine oxidase-A ratio increased during development both in the substantia nigra and the striatum. The significance of these changes is discussed.