Vitamin D3 inhibited the promotion by exogenous promoters in experimental colonic tumorigenesis. To give more insight into this phenomenon, the effect of 1 alpha-hydroxyvitamin D3 (1 alpha(OH)D3) on N-methyl-N-nitrosourea (MNU)-induced colonic tumorigenesis was studied in rats without exogenous promoters. Fecal bile acids were analyzed to examine as to whether 1 alpha(OH)D3 increased the concentration of soluble bile acids. Eighty-seven female F344 rats received 2 mg of MNU intrarectally 5 times in 2 weeks, and were divided into 3 groups. One group (n = 29) was left without any treatment. Two groups (each, n = 29) were given 0.2 ml of medium chain triglyceride (MCT) or MCT containing 0.04 microgram of 1 alpha(OH)D3 through an intragastric route thrice weekly for 38 weeks. At autopsy, numbers of rats with colonic tumor were 9 (31%), 10 (34%) and 10 (34%) in the group receiving MNU alone, MNU + MCT and MNU + 1 alpha(OH)D3, respectively (chi 2 = 0.103, P less than 0.95). Fecal bile acid profiles showed no appreciable difference among these groups, nor was observed any increase of soluble bile acids in the MNU + 1 alpha(OH)D3 group. These results indicated that the administration of 1 alpha(OH)D3 did not affect colonic tumorigenesis under the condition where exogenous promoters were not applied, and that 1 alpha(OH)D3 did not seem to interfere the formation of bile acid calcium salts in animals on a regular diet.