Casein kinase 1ε (CK1ε) is a serine/threonine protein kinase that has been suggested to participate in the regulation of various signaling pathways. In this report, we examined the tissue distributions of three putative alternatively spliced forms of rCk1ε by RT-PCR. This analysis confirmed that all three isoforms are expressed in rat tissues with different tissue-specific expression patterns. Immunohistochemical analysis showed that the intracellular distribution of rCK1ε-3 in neurons was broader than that of rCK1ε-1. Moreover, the kinase activity of the rCK1ε-3 protein differed from that of rCK1ε-1. These data suggest that rCK1ε-1 and rCK1ε-3 may play different functional roles.