The incorporation of a biologically active group into a naturally occurring structure is an interesting approach to new pharmacological agents. For example, when an acetylcholine-like structure is incorporated in ring-A of 5a-androstane-17-one or 5a-pregnan-20-one, neuromuscular blocking agents of up to one-fifteenth the potency of tubocurarine are obtained (Lewis, Martin-Smith, Muir & Ross, 1967). Although of limited clinical value, such compounds present useful leads for further chemical exploration, particularly the synthesis of bisquaternary amino-steroidal salts. A series of 2,8,16,3-diamino-5a-androstane-3a,17p-diol dimethohalide derivatives was described by Buckett, Hewett & Savage (1967). One of them, 2ft,16,/-dipiperidino-5a-androstane3a,17,8-diol diacetate dimethobromide (code number Org.NA97; approved name pancuronium bromide), is a potent neuromuscular blocking agent and in the experiments described in this paper its actions were compared with those of tubocurarine chloride. Cats were anaesthetized with a mixture of chloralose (70 mg/kg) and sodium pentobarbitone (12 mg/kg) injected intraperitoneally; rabbits and dogs were anaesthetized with sodium pentobarbitone (45 mg/kg) injected intravenously, and hens with sodium barbitone (200 mg/kg) injected intravenously. In all species the trachea was intubated and artificial ventilation applied throughout each experiment Twitches and tetani of a gastrocnemius muscle were elicited by stimulating the peripheral portion of the crushed sciatic nerve in the popliteal space with rectangular shocks of