Administration to mares of the anabolic steroid, methandriol, at the maximum recommended dose (300 mg every 3 weeks) for 1 1/2 years had no effect on reproductive characteristics except for suppression of GnRH-induced LH release and a tendency to suppress basal LH levels and the height of the ovulatory LH surge. A 4-fold increase in dosage caused marked suppression of basal LH, the LH surge, and GnRH-induced LH release. Other reproductive responses were minimally affected. There were no behavioural effects, and no changes in weight occurred when mares were compared with matched controls. Small and moderate doses of testosterone induced behavioural changes without affecting reproductive function. However, large doses, which raised plasma testosterone to levels similar to those of stallions, eventually caused total suppression of all reproductive activity within 1 month and the development of markedly vicious stallion-like behaviour. Apart from the aggression all changes disappeared within 1 month after the end of treatment.