The ontogeny of a dopamine and cyclic adenosine-3':5'-monophosphate-regulated phosphoprotein with an apparent molecular weight of 32 kilodaltons (DARPP-32) has been studied in the central nervous system of the prenatal, newborn and adult mouse. DARPP-32-immunoreactive somata were first identified at day 12 of gestation, in the primary olfactory cortex and in the ventrolateral medulla oblongata. On day 14 of gestation, neurons containing DARPP-32-like immunoreactivity became apparent in the caudate nucleus, olfactory tubercle, nucleus accumbens, frontoparietal cortex and the ventral medulla oblongata. During the period up to and including birth, the number of cell bodies and fibres in all these areas increased markedly. In addition, DARPP-32-positive neurons became visible in the olfactory nucleus, the arcuate nucleus, and DARPP-32-positive cells appeared in the choroid plexus of the lateral, third and fourth ventricles. DARPP-32-containing fibres could be seen in the median eminence, the ventrolateral thalamus, and in the striatonigral projection, descending in the internal capsule to ramify extensively in the substantia nigra. Only in the cerebellum and suprachiasmatic nucleus did the development of DARPP-32-like immunoreactivity occur postnatally. The development of tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis, was simultaneously examined. The arrival of the tyrosine hydroxylase-containing projection to the caudate nucleus, the olfactory tubercle and the nucleus accumbens apparently occurred 1-2 days after the appearance of DARPP-32-immunoreactive cells within these regions. In the ventral and ventrolateral medulla oblongata, and the primary olfactory cortex, no tyrosine hydroxylase innervation was seen near the DARPP-32-positive neurons at days 12-14. The organization of the DARPP-32-containing somata of the caudate nucleus into aggregates of 5-15 neurons was partly paralleled spatially by an increased density of tyrosine hydroxylase-positive fibres. Many DARPP-32-immunoreactive cells in the immature mouse brain are present by the day of birth, particularly in the areas known to receive a dopaminergic innervation. The development of these presumptive dopaminoceptive DARPP-32-containing neurons does not seem to be dependent on the presence, however, of a dopaminergic input, since in all regions examined DARPP-32-LI preceded the appearance of tyrosine hydroxylase-like immunoreactivity by at least 1-2 days. Indeed, the results suggest that the existence of DARPP-32-like immunoreactivity in cell bodies and dendrites may be a pre-requisite for the formation or subsequent stabilization of dopaminergic synapses.