The earliest detection of the proenkephalin gene was on embryonic day 16 in neuronal cell bodies in the ventrolateral portion of the caudal neostriatum. This expression was identified by both immunocytochemistry for synenkephalin, the nonopioid N-terminus of proenkephalin (1-70), and preproenkephalin in situ hybridization with a complementary DNA probe. Two developmental gradients of preproenkephalin expression and synenkephalin immunoreactivity were observed: (i) a ventrolateral to dorsomedial and caudal to rostral gradient in the rostral caudate-putamen; and (ii) a ventromedial to dorsolateral and rostral to caudal gradient in the caudal caudate-putamen. Ventrolateral to dorsomedial and caudal to rostral developmental gradients of synenkephalin fiber immunoreactivity were also identified in the globus pallidus. Methionine enkephalin immunoreactivity was not consistently detectable until postnatal day 10 and 15 in the rostral and caudal globus pallidus, respectively. A transient patchy distribution of increased preproenkephalin expression from embryonic day 20 through postnatal day 5 occurred. These patches and a subcallosal streak were found to overlap partially with areas of increased tyrosine hydroxylase immunoreactivity by adjacent section analyses. The earliest detection of tyrosine hydroxylase immunoreactivity was found to coincide with that of proenkephalin on embryonic day 16, but in differing regions of the corpus striatum. Tyrosine hydroxylase immunoreactivity in the rostral caudate-putamen preceded, while in the caudal caudate-putamen it followed first expression of the proenkephalin gene. Early proenkephalin expression, by both synenkephalin immunocytochemistry and preproenkephalin in situ hybridization, was also detected in the central nucleus of the amygdala on embryonic day 16 immediately ventral to the area of expression in the caudate-putamen. Preproenkephalin expression in the olfactory tubercle and nucleus accumbens first appeared on embryonic day 20 and expression proceeded in a lateral to dorsomedial gradient continuous with the ventral part of the rostral caudal-putamen. Relatively late detection of methionine enkephalin immunoreactivity in comparison to synenkephalin possibly indicates a developmental delay in the complete enzymatic processing of the proenkephalin precursor. Differing gradients in the ontogeny of preproenkephalin expression in the rostral vs the caudal caudate-putamen suggest possible anatomical and developmental differences of these two regions. Also, transient compartmentalization of preproenkephalin expression and differences in dopaminergic innervation as detected by tyrosine hydroxylase immunoreactivity were further support for the existence of two subsets of proenkephalinergic neurons in the caudate-putamen. Contemporaneous development of preproenkephalin expression and synenkephalin immunoreactivity in the central nucleus of the amygdala with the ventral part of the caudal caudate-putamen also suggested developmental homology.(ABSTRACT TRUNCATED AT 400 WORDS)