In sagittal brain sections of newborn male rats (1-day-old) there were no regional differences in the IC50 values of dopamine at [125I]iodosulpride binding sites. In contrast, in 20- and 60-day-old rats, there was a selective increase in the IC50 values of dopamine in the nucleus accumbens, caudate-putamen, and olfactory tubercule. The IC50 values of these regions decreased in 262-day-old rats. Some of the other brain areas appeared to behave in a similar, but much less pronounced, fashion. Thus, there were significant regional differences in the IC50 values of young, adult, and old rats. In addition, there was a rapid increase in [125I]iodosulpride binding between the newborn and the 20-day-old rats, which leveled off thereafter, and selectively decreased in the substantia nigra of the 262-day-old rats. In conclusion, these results indicate that a biphasic decrease-increase in the affinity of D2 agonist binding sites occurs selectively in the basal ganglia. These findings may be of relevance for developmental diseases in which dopaminergic mechanisms have been implicated, such as schizophrenia and Parkinson's disease.