D1 dopamine receptor (D1R) and DARPP-32 (a dopamine and adenosine 3',5'-monophosphate regulated phosphoprotein), gene expression was studied in the rat striatum in adults and during ontogeny by in situ hybridization. D1R mRNA was first detected in the striatal primordium at day 17 of gestation. At day 18, D1R mRNA was found throughout the striatum. Before birth, the striatal neurons had neuroblastic aspect and were close together, giving homogeneous and compact labelling. After birth, the topography and aspect of the neurons containing D1R mRNA and DARPP-32 mRNA were similar. The two mRNAs were detectable in the caudate-putamen, accumbens nucleus and olfactory tubercle. The microautoradiographic analysis demonstrated that D1R and DARPP-32 genes are massively expressed by the medium-sized striatal neurons. The proportion of medium-sized neurons containing the DARPP-32 mRNA was however higher than that of the neurons containing the D1R mRNA. Furthermore, an unexpected proportion of large-sized neurons express these genes. This proportion varies with development. Comparison between the appearance, topography and frequency of choline-acetyltransferase immunoreactive neurons and large-sized neurons containing D1R or DARPP-32 mRNA suggest that these large-sized neurons containing D1R and DARPP-32 mRNAs are cholinergic ones.