Three groups of Wistar rats were fed a diet containing aminopyrine (0.2%) and NaNO2 (0.2%) and either tap-water (controls), sea buckthorn juice or ascorbic acid solution ad libitum for 38 weeks. All 17 rats given tap-water developed tumours in the liver, and six developed tumours in the lungs and four in the kidneys; in rats given sea buckthorn juice, 15 had tumours of the liver, 11 of the lungs and two of the kidneys; the incidences in rats given ascorbic acid were 18/18, 6/18 and 4/18, respectively. The average life span of the group given sea buckthorn juice was 270 days, which was significantly longer (p less than 0.01) than those of rats given tap-water (195 days) or ascorbic acid (220 days). Microscopic examination of the livers of rats receiving sea buckthorn juice showed fewer foci of carcinogenesis than those of the control and the ascorbic acid groups. The results suggest that sea buckthorn juice can block the endogenous formation of N-nitroso compounds more effectively than ascorbic acid and thereby prevent tumour production.