Neurons being generated in the striatum of 10 fetal cats were pulse labeled by injection of [3H]thymidine directly into the maternal uterus at times ranging between the 22nd and 30th days (E22-E30) of the 65-day gestational period. Many of the striatal neurons labeled during this interval were found, at adolescence, to form 100- to 600-mum-wide cell clusters in the caudate nucleus. In E24-E30 specimens, we compared the distributions of these cell clusters with the locations of patches of low acetylcholinesterase activity and high enkephalin immunoreactivity (the "striosomes') visualized in serially adjoining sections. We found precise matches between most of the cell clusters and the acetylcholinesterase-poor enkephalin-rich zones, regardless of the embryonic age at which exposure to the [3H]thymidine had occurred. We conclude that the histochemically distinct striosomal patch-works observed in the acetylcholinesterase and enkephalin preparations correspond to ontogenetic units of the striatum.