In the rat heart, monoamine oxidase (MAO)-B activity was shown to predominate in 2- to 3-week-old animals, whereas MAO-A activity was reported to be very low in newborn rats and to increase considerably with age until it predominates. These results are in contrast with those found in the mouse heart, where an age-dependent increase in MAO-B activity with no changes in 5-hydroxytryptamine deaminating activity was found to occur. There is evidence that the adult values of MAO activity are reached early in development in rat kidney and liver. In the rat lung the adult values of MAO-A activity are reached by day 40, whereas MAO-B activity is still increasing by day 80. Important differences have been reported in the developmental pattern of the two forms of MAO in the rat and mouse brain, with a decrease in the MAO-A/MAO-B ratio during postnatal development. In the human brain, the ontogenetic development of MAO-A and MAO-B appears to parallel that observed in the rodent brain. It is worth noting that most of the available data have to be considered with reservation owing to many methodological problems. Further studies are clearly needed to get reliable information on the ontogenesis of MAO in mammalian tissues.