Oestrogen plays an important role in testicular function. This study used mice null for oestrogen receptor alpha (ER alpha) or beta (ER beta) to investigate which receptor mediates the effects of oestrogen within the testis. Groups of ER alpha knockout mice (alpha ERKO) and ER beta knockout mice (beta ERKO) and wild-type littermates (n=5-8) were killed at 11 weeks post partum. One testis was fixed in Bouin's fluid for stereology and the other frozen for testosterone measurement. Trunk blood was collected for testosterone RIA. The optical disector combined with the fractionator methodology was used to estimate Leydig, Sertoli and germ cell numbers. At all times, the knockout animals were compared with their wild-type littermates. The physical disector quantified cells stained immunohistochemically for the apoptotic marker active caspase-3 and Hoechst staining was used to identify nuclear fragmentation. The mean Leydig cell volume was measured using the point sampled intercept method. The Leydig cell number per testis was significantly increased in beta ERKO mice but not in alpha ERKO mice. Plasma and testicular testosterone concentrations were increased in alpha ERKO mice but no changes were observed in beta ERKO mice. Hypertrophic Leydig cell changes were observed in alpha ERKO mice, and a decreased mean cell volume was seen in beta ERKO mice. No difference in Sertoli cell number per testis was observed in any of the groups. The spermatogonial cell number per testis was increased in beta ERKO mice. Immunohistochemistry identified increased numbers of active caspase-3-labelled germ cells per testis in alpha ERKO mice but not beta ERKO mice. Hoechst staining supported these findings. There was significant germ cell loss in alpha ERKO mice. This study suggests that ER beta may be involved in regulation of Leydig cell proliferation and testosterone production in the adult mouse testis.