Male BC3F1 hybrid mice were injected with 10 micrograms oestradiol benzoate/day alone or in conjunction with 80 micrograms testosterone propionate/day. Oestrogen alone decreased the weights of the accessory sex organs, but oestrogen + testosterone resulted in an increase. Assessment of spermatogenesis by a variety of methods revealed little effect of the hormones. Testicular weight and the histological appearance of the seminiferous epithelium was not altered appreciably, although Leydig cell atrophy was apparent when testosterone was given. The kinetics of spermatogenesis, studied by cell separation techniques and biochemical features of spermatogenesis, including protamine biosynthesis, lactate dehydrogenase levels and the formation of the highly resistant, condensed sperm nucleus, were also unaffected by the hormonal treatments. However, a reduction in the number of intact testicular spermatozoa in mechanically prepared cell suspensions was obtained. It is concluded that, in these animals, spermatogenesis is relatively insensitive to the administration of oestrogen.