Prostaglandins E[PGE] and F were measured to the testis, epididymis, vas deferens and seminal vesicles of CD-1 mice from 2 to 8 wk of age. The concentration of PGF was higher than that of PGE in all organs studied, except for the vas deferens. The concentration of PG was age-dependent, showing a progressive decline from immaturity to adulthood. However, in the testis, there was an apparent transient increase in the concentration of PG at the age of 6 wk. The concentration of both PG in the seminal vesicle changed very little between the ages of 5 and 8 wk. The vas deferens had a significantly higher PG concentration than any of the organs studied, and a unique pattern of changes in the levels of PGE and PGF with age. In the vas deferens of 2- and 3 wk old mice, the concentration of PGF was higher than the concentration of PGE, but after 4 wk of age PGE became somewhat more abundant than PGF. Treatment of immature mice with testosterone propionate (TP) produced significant changes in PG concentrations, resulting in PG levels resembling those of adult animals. The treatment also changed the ratio of PGE to PGF in the vas deferens (from 1:2 to 1:1). Hereditary dwarf mice had higher levels of PG in the tissues of the male reproductive system than did their normal littermates. The treatment of dwarf mice with TP generally reduced the concentration of PG in their reproductive system and resulted in a PG pattern more characteristic of normal adult males of the same strain. The data demonstrate pronounced changes in PG levels in the tissues of the male reproductive system of mice during sexual maturation. These changes can be accounted for by an increase in testicular testosterone secretion.