Two populations of aspiny interneurons have been identified in the mammalian striatum, one cholinergic and the other using the neuropeptide somatostatin as a neurotransmitter. The times at which these 2 cell populations undergo their final mitosis were studied by injecting tritiated thymidine into timed pregnant rats and then processing the brains of the progeny as young adults for immunohistochemistry with monoclonal antibodies to choline acetyltransferase and somatostatin followed by autoradiography. Choline acetyltransferase-immunoreactive neurons became postmitotic in a caudal-to-rostral gradient; the occurrence of final mitosis was maximal on embryonic day (E) 12 at the most caudal level and on E15 at the most rostral. A more subtle lateral-to-medial gradient was also observed in the precommissural striatum. In contrast, no obvious gradients were seen with somatostatin-immunoreactive neurons; regardless of their location within the striatum, these neurons underwent their final mitosis on days E15-16, towards the end of cholinergic neurogenesis. These results indicate that although both cholinergic and somatostatin-containing cells represent interneuronal populations in the striatum, they display distinctly different spatiotemporal patterns of neurogenesis.