The anti-secretory activity of the competitive H2-antagonist HUK 978 was determined in rat, guinea-pig and dog. In all systems examined, HUK 978 was more potent than cimetidine and ranitidine both intravenously and orally. In addition, the compound at approximately equipotent doses as these established H2-antagonists exhibited a significantly longer inhibitory profile following oral and systemic administration. Data from these pharmacological studies and the in vitro investigations previously reported, suggest that HUK 978 is a highly specific H2-antagonist and inhibits acid secretion for longer periods than other competitive compounds.