In order to promote better understanding of the physiological roles of prostaglandin F(2alpha) in the mouse testis, we investigated the protein expression and the cellular localization of the enzymes cyclooxygenase and prostaglandin F synthase that are essential for the production of prostaglandin F(2alpha), and the binding site, which is the prostaglandin F(2alpha )receptor (FP). Western blot exhibited the expression of FP protein in wild type mouse testis, and that of prostaglandin F synthase and cyclooxygenase-1 proteins in the both of wild type mouse and FP-deficient mouse testes. The expression of prostaglandin F synthase and cyclooxygenase-1 were detected intensely in Leydig cell-rich fraction, and that of FP was detected equally in Leydig cell-rich fraction and the other fraction. Immunohistochemistry for cyclooxygenase-1 and prostaglandin F synthase demonstrated their co-localization in mouse Leydig cells. Histochemistry for FP demonstrated the localization in Leydig cells and in spermatids of seminiferous tubules. Double histochemical staining confirmed the co-localization of cyclooxygenase-1, prostaglandin F synthase and FP in the Leydig cells. These findings indicate that prostaglandin F(2alpha) may have an effect on the functions of Leyding cells in an autocrine fashion. It implies that prostaglandin F synthase and FP are involved in the control of testosterone release from Leydig cells and in spermatogenesis via the local pathway and the hypothalamo-hypophysial-testis pathway, and affect the testicular function.