The Leydig cells of young hypophysectomized rats are highly sensitive to the stimulatory effects of exogenous pituitary hormones. The aim of this study was to analyse the role of testicular macrophages in the response of Leydig cells to different hormones. Male rats were hypophysectomized at 28 days of age and 10 days later they were injected intratesticularly with dichloromethylene diphosphonate-containing liposomes (right testis) to deplete testicular macrophages, and with 0·9% NaCl (left testis). One week later, the animals were treated daily with 1 IU rat GH (rGH)/rat, 5 IU recombinant human FSH (recFSH)/rat, 10 IU human chorionic gonadotrophin (hCG)/rat, or vehicle for 7 days. The animals were killed on the day after the last injection. The animals treated with rGH showed increased body weight and increased number and size of testicular macrophages in the left testes, but no significant effects on Leydig cells were found. Treatment with recFSH induced a significant increase in testicular weight and tubular diameter in both testes. In the left testes, the number and size of macrophages were increased; the number of Leydig cells was not changed, although they showed a significantly increased cross-sectional area. This effect was abolished in the right (macrophage-depleted) testes. However, the effect of recFSH on the growth of the seminiferous tubules was not modified by the absence of macrophages. Rats treated with hCG showed increased testicular weight and serum testosterone levels, as well as an increased weight of the ventral prostate. In the left testes, the number and size of both macrophages and Leydig cells were increased. Otherwise, the number of Leydig cells was unchanged in the absence of macrophages, whereas the increase in the size of Leydig cells was partially abolished. These data indicate that testicular macrophages are needed for the response of Leydig cells to gonadotrophin treatment.