Summary. Intravenous injection of oxytocin into rams immediately before collection of semen with an artificial vagina, resulted in an increase in the volume of semen and the number of spermatozoa per ejaculate. After 6 weeks, during which the rams received 35 i.u. oxytocin per week, the volume of semen and the number of spermatozoa per ejaculate declined to the level of the non-treated rams and this decline was maintained for the next 4 weeks. The percentage of abnormal spermatozoa in semen from rams treated with oxytocin increased significantly (PP<0·05), with a maximum response at 10 i.u. oxytocin. Oxytocin appeared to have an immediate stimulating effect on the ejection of spermatozoa and seminal plasma during emission and a long term adverse effect on spermatogenesis.