Merino ram lambs were actively immunized against oestradiol-6 (o-carboxy methyl) oxime-BSA conjugate at 14 weeks of age and received a booster injection 4 weeks later. This treatment led to an increase in plasma concentrations of gonadotrophin and tended to enhance the increase in testicular volume until 26 weeks of age; however, testis size and mass at time of castration (30 weeks of age) were similar to values in BSA-immunized lambs. Detrimental effects were observed in some oestradiol-immunized ram lambs, for example a steep decline in testicular volume towards the end of the experiment, the presence of large vacuoles within the seminiferous epithelium and, in one lamb, few germ cell at 30 weeks of age. Testicular blood plasma flow was significantly reduced in oestradiol-immunized lambs (P < 0.01). The steroidogenic function of the testis was markedly enhanced in oestradiol-immunized lambs as reflected by high plasma concentrations of testosterone measured at 22, 26 and 30 weeks of age and by high testosterone production calculated from blood flow and venous-arterial differences at 30 weeks of age. Nevertheless, total live mass gain over the 16 week study was not increased in oestradiol-immunized lambs. Testicular biopsies were taken at 22 and 26 weeks of age in half of the lambs in each treatment group. Testicular volume measured at castration was decreased in control lambs in which biopsies were taken (P < 0.05), and plasma concentrations of testosterone measured at 30 weeks of age were significantly lower in oestradiol-immunized lambs in which biopsies were taken (P < 0.02) compared with lambs in which no biopsy had been taken. It is concluded that active immunization against oestradiol in ram lambs does not advance the time of onset of puberty and does not confer any reproductive or maturational advantages.