D-type G1 cyclins are known to be crucial for the progression of mitotic cell cycle in mammals. Although many studies have been performed to elucidate the roles of D-type cyclins, it is largely unknown whether D-type cyclins are directly involved in the regulation of meiotic germ cell development. In the present study, we examined the expression patterns of D-type cyclins (cyclin D1 and D3) during male germ cell development by northern blot and in situ Hybridization analyses. In the adult testes, we detected a 4.2 kb cyclin D1 mRNA and two different sizes (2.3 kb and 1.8 kb) of cyclinD3 mRNAs. The short form of the cyclin D3 transcript was testis-specific. Along with the testicular development, expression of cyclin D3 mRNA was increased whereas cyclin D1 mRNA was gradually decreased. in situ hybridization study also revealed that the expression of cyclin D3 was restricted to the postmeiotic germ cells. Furthermore, the 2.3 kb transcript was highly expressed in the round spermatids and decreased in the elongated spermatids/residual bodies, while the 1.8 kb transcript was expressed in elongated spermatids/residual bodies more abundantly. Sucrose-gradient separation of polysomal RNA fractions demonstrated that some portions of the 2.3 kb transcript are translationally active, while the 1.8 kb transcript is likely to be inactive. Taken together, the present data suggest a functional importance of cyclin D3 expression in the differentiated postmeiotic male germ cells.