Marijuana smoke and cannabinoids adversely affect male reproductive function in human and rodent through the cannabinoid receptors. To understand the possible function of cannabinoid receptor 1 (CB1) in spermatogenesis, expression of CB1 in testis during the postnatal development was examined in mice. Semiquantitative RT-PCR analysis revealed that testicular CB1 mRNA level was relatively high at 1 week post partum (p.p.). Following decrease during prepubertal development (2 weeks p.p.) and CB1 mRNA level re-increased during puberty (4 weeks p.p.) and reached the peak in adult testis. At 1 week p.p., some spermatogonia and Leydig cells showed strong immunoreactivity of CB1. At 2 weeks p.p., CB1 immunoreactivity was largely found in the primary spermatocytes as well as spermatogonia, and Leydig cells showed a weak signal. In adult testis, strong immunoreactivity was found in Leydig cells and luminal epithelia of seminiferous tubule. Germ cells including spermatozoa were positive for CB1 immunoreactivity. On Western blot, multiple forms of CB1 proteins were detected in testes, suggesting oligomerization of CB1. Ubiquitous, but spatiotemporal difference in expression of CB1 in soma and germ line during postnatal development of testis suggests functional involvement of CB1 signaling in steroidogenesis, spermatogenesis and fertilization.