The present study examined immunohistochemically the CNS distributions of a splice variant of the mu-opioid receptor, MOR-1D, in both rats and mice. In MOR-1D, exon 4 of MOR-1 is replaced by two additional exons that code for seven amino acids. Using rabbit antisera, we compared immunohistochemically the regional distribution of a C-terminal epitope of MOR-1D to that of a C-terminal epitope from MOR-1 and a C-terminal epitope from another splice variant, MOR-1C. The general distribution of MOR-1D-like immunoreactivity was similar in both mouse and rat. MOR-1D-like immunoreactivity was seen in the dentate gyrus and in the mossy fibers of the hippocampal formation, the nucleus of the solitary tract and the area postrema, the inferior olivary nucleus, the nucleus ambiguous, the spinal trigeminal nucleus and the spinal cord. MOR-1D-like immunoreactivity was not observed in some regions containing dense MOR-1-like immunoreactivity, such as the striatum or the locus coeruleus. In regions containing MOR-1, MOR-1C and MOR-1D, the pattern of each variant was unique.MOR-1D and MOR-1C are splice variants of the cloned mu-opioid receptor MOR-1. Although they differ only at the tip of the carboxy terminus, they show marked differences in their regional distributions, as determined immunohistochemically by epitopes in their unique carboxy termini. Since the splice variants are derived from the same gene, these differences in regional distribution imply region-specific messenger RNA processing.