GPR88 is a putative G-protein-coupled receptor that is highly and almost exclusively expressed in the striatum. Its function remains unknown. We investigated GPR88 cellular localization and expression levels across development in different functional domains of the striatum in juvenile (P25), adolescent (P40), and adult (P70) rats, by in situ hybridization histochemistry. At all ages, GPR88 mRNA expression was most robust in the sensorimotor (lateral) striatum and was detected in virtually every neuron. Expression was highest in juveniles and decreased thereafter with regionally distinct trajectories. Thus, in the dorsal striatum, there was a progressive decrease from juveniles to adolescents to adults. In contrast, in the nucleus accumbens, the only (modest) decrease occurred between juveniles and adolescents. These findings indicate that GPR88 is expressed in all striatal neurons, but is differentially regulated across development in different striatal regions.