Abstract Pathological-anatomical results on the effect of coumarin in animal experiments 5 groups of rats were fed in chronic feeding studies extending over a period of up to 2 years a diet to which coumarin (1,2-benzopyrone) had been added in concentrations of 0.1% (group 1), 0.25% (group 3), 0.5% (group 2) or 0.6% (groups 4 and 5). The groups consisted of 20 male and 20 female rats of each of the groups 1 and 2, 25 male and 25 female rats of each of the groups 3 and 4 or 32 male rats of the group 5. Control rats: 40 rats to groups 1 and 2, 50 rats to group 3, 4 and 5. 14 rats from the group 2 which had received 0.5% coumarin in their diet and 25 rats from the two groups 4 and 5 that had been fed 0.6% coumarin survived until the end of the experimental period. In the group that had been fed 0.5% coumarin, 12 of the 14 surviving animals developed liver carcinomas; in the two groups that had received 0.6% coumarin, but with a reduced uptake of food, 5 of the 25 survivors had carcinomas. The rats in the groups fed 0.1% or 0.25% coumarin in their diet exhibited only some benign adenomas or proliferations of the bile ducts. Gross autopsy revealed a number of small to massive greyish-white tumours with a nodular and uneven surface in different areas of the liver. Microscopically, all carcinomas corresponded to an identical type — that of bile duct carcinoma. The histological findings are described in detail.