SUMMARY Injection of a 'rapid-acting' preparation of porcine adrenocorticotrophic hormone (ACTH) into three boars resulted in a rapid rise in plasma testosterone levels which accompanied the expected rise in plasma corticosteroids. Urinary dehydroepiandrosterone (DHA) levels were measured in one boar and were found to be raised also. The results suggest that the effect involved enhanced testicular steroid activity and was related to the dosage of ACTH employed. This action of ACTH is thought to be mediated through the adrenal cortex since injection of cortisol elicited a rise in testosterone similar to that observed after injection of ACTH. ACTH had no effect on testosterone levels in a castrated boar. When a 'longacting' preparation of ACTH was administered to two boars twice daily for 5 days, testosterone levels were depressed. It was concluded that ACTH may bring about an increase or a decrease in plasma testosterone levels in the boar depending upon the length of time increased levels of ACTH are present in the circulation.