Rab3A, a ras p21-related small GTP-binding protein, is implicated in the exocytosis of neurotransmitters. Recently, Rabphilin-3A, a putative target protein for Rab3A, was identified and its cDNA has been cloned from bovine brain. In this study, we isolated a cDNA encoding a mouse Rabphilin-3A homolog from the insulin-secreting cell line, MIN6. Mouse Rabphilin-3A is a protein of 681 amino acids exhibiting overall 88.5% identity with bovine Rabphilin-3A. The amino acid identity between mouse and bovine Rabphilin-3A is highest in their carboxyl-terminal halves (97.8% identity) and amino-termini (93.0% identity), which contain the region of the two internal repeats homologous to the regulatory domain (C2 domain) of protein kinase C and the putative Rab3A-binding region, respectively. RNA blot analysis revealed that Rabphilin-3A mRNA is expressed in endocrine and hormone-secreting clonal cells, including rat adrenal glands, MIN6, the hamster insulin-secreting cell line, HIT-T15, and the rat catecholamine-secreting cell line, PC12, as well as rat brain. These results suggest that Rabphilin-3A might be involved in the exocytosis of secretory vesicles in hormone-secreting cells as well as in neurons.