Oestrogen is a metabolite of testosterone, but its role in spermatogenesis is ill-defined. Oestrogen may exert its effects on spermatogenesis, as oestrogen receptor (ER)-beta has been localised to both germ and somatic cells. This study sought to establish whether the restoration of early germ cell numbers in spermatogenesis by high-dose exogenous testosterone was influenced by its metabolite, oestrogen. The ER antagonist (ICI 182780) was administered, at a dose known to impair oestrogen action in the male reproductive tract, during testosterone treatment of gonadotrophin-releasing hormone (GnRH)-immunised rats, and germ cell numbers were determined. GnRH-immunised adult Sprague-Dawley rats (n=7-8 per group) received two doses of testosterone, either as a Silastic implant (24 cm (T24 cm)) or an injectable ester for 10 days alone or in combination with ICI 182780 (2 mg/kg, s.c. injection daily). Control rats received vehicle alone. Testes were perfusion-fixed and germ cells were quantified by the optical disector technique. GnRH-immunisation reduced (P<0.001) both type A/ intermediate spermatogonial and type B spermatogonial/ preleptotene spermatocyte number (56% of control) and leptotene/zygotene spermatocyte number (63% of control). Pachytene spermatocyte and round spermatids were reduced to 12% and l% (P<0.01) of control respectively. Testosterone treatment did not increase type A/intermediate spermatogonial number compared with GnRH-immunised controls over the 10-day study period. Treatment with testosterone-esters increased type B spermatogonial/preleptotene spermatocytes and leptotene/zygotene spermatocyte numbers (both being ~83% of control, P<0.05), while T24 cm treatment did not significantly increase their numbers (~73% of control) compared with GnRH-immunised controls. Both treatments increased pachytene spermatocyte and round spermatid numbers to 55% and 8% of control respectively. Co-administration of ICI 182780 had no effect on any of these germ cell numbers. We conclude that oestrogen action plays no role in the short-term restoration of spermatogenesis by testosterone in the GnRH-immunised rat.