The role of the gonadotrophins in regulating numbers of Leydig and Sertoli cells during fetal and postnatal development was examined using normal mice and hypogonadal (hpg) mice, which lack circulating gonadotrophins. The disector method was used to determine the number of cells from day 16 of gestation until adulthood. The numbers of Leydig cells did not change significantly between day 16 of gestation and day 5 after parturition in normal mice and were not significantly different from numbers in hpg mice at any age up to day 5 after parturition. There was a 16-fold increase in the number of Leydig cells in normal mice between day 5 and day 20 after parturition, followed by a further doubling of number of cells between day 20 and adulthood. The number of Leydig cells in hpg testes did not change between day 5 and day 20 after parturition but doubled between day 20 and adulthood so that the number of cells was about 10% of normal values from day 20 onwards. Leydig cell volume was constant in normal animals from birth up to day 20 and then showed a 2.5-fold increase in adult animals. Leydig cell volume was normal in hpg testes at birth but decreased thereafter and was about 20% of normal volume in adult mice. The number of Sertoli cells increased continuously from day 16 of gestation to day 20 after gestation in normal mice and then remained static until adulthood. The number of Sertoli cells in hpg testes was normal throughout fetal life but was reduced by about 30% on day 1 (day of parturition). Thereafter, Sertoli cells proliferated at a slower rate but over a longer period in the hpg testis so that on day 20 after parturition the number of Sertoli cells was about 50% of normal values, whereas in adult mice the number was 65% of normal. The number of gonocytes did not change between day 16 of gestation and day 1 and did not differ between normal and hpg testes. The number of gonocytes increased nine-fold in normal testes but only three-fold in hpg testes between day 1 and day 5 after parturition. Gonocytes differentiated into spermatogonia in both normal and hpg testes between day 5 and day 20 after parturition. These results show: (i) that fetal development of both Sertoli and Leydig cells is independent of gonadotrophins; (ii) that normal differentiation and proliferation of the adult Leydig cell population (starting about day 10 after parturition) is dependent on the presence of gonadotrophins; and (iii) that the number of Sertoli cells after birth is regulated by gonadotrophins, although proliferation will continue, at a lower rate and for longer, in the absence of gonadotrophins.