Human metabolic studies have suggested a positive association between dietary intake, faecal bile acid excretion and the development of colon cancer. Similar investigations in experimental models of this disease have also implicated faecal bile acids but in both animals and man the results remain equivocal. This study sequentially examines the outputs and concentrations of faecal bile acids in dimethylhydrazine (DMH) treated groups of rats (n = 10), killed at 5-weekly intervals from the 10th to 40th week following the first injection. The sequential results showed that both the output and concentration of faecal bile acids decreased with time and accompanied an increase in both the incidence and numbers of colonic neoplasms over the 40 weeks of the study. When the animals were grouped according to the histological classification of their tumours, the faecal bile acids did not differ between animals with and without tumours. Further, when the latter group were sub-divided into those with adenomas alone and those with carcinomas, faecal bile acid outputs and concentrations did not differ between them, nor when they were compared with the tumour-free animals. The results of this study do not support a role for faecal bile acids in the dimethylhydrazine-induced model of experimental colon cancer in rats.