With increasing frequency, psychotropic medications are being prescribed to young children, often for long periods of time. The interaction between psychotropics and the developing brain has not been systematically investigated in humans. Data collected from animals suggest that developing neurotransmitter systems can be exquisitely sensitive to early inhibition or stimulation by pharmacological agents, which can lead to permanent changes in adult life. Most of these data are collected from rodents, and their extrapolation to humans is difficult. More relevant models could be developed for instance using primates. In humans, the focus of research has traditionally been on the possible teratogenic effects of prenatal drug exposure. Recently introduced quantitative imaging techniques can offer new approaches to studying the effects of psychotropics on the developing brain. This research has clear implications for the safety and efficacy of psychopharmacologic drug in children.