The mechanism by which calcium may protect against colon carcinogenesis was studied in rats fed three levels of calcium (0.5, 1.0 and 1.5 g/100 g diet) and a high butter diet (20 g/100 g). Results were compared with values obtained using a low butter diet (5 g/100 g) supplemented with 0.5 g Ca/100 g diet. Concentration and relative proportions of individual fecal bile acids were determined by gas chromatography, and in vivo colonic cell proliferation was assessed using [3H]thymidine. Total fecal output and total fecal lipids increased as dietary calcium increased. When calcium supplementation remained at 0.5%, raising the level of fat in the diet increased the fecal concentrations of deoxycholate, omega-muricholate and total bile acids. However, when the level of fat supplementation remained constant at 20 g/100 g diet, but dietary calcium was 1.0 or 1.5 g/100 g, the concentration of these bile acids returned to the level found in feces of rats fed the low fat diet. Despite the significant decrease in the concentrations of certain fecal bile acids with increasing calcium supplementation, these differences were not reflected in substantive changes in epithelial cytokinetics. These data show that in rats fed high butter diets, 1.0% calcium is sufficient to lower total bile acid concentration and the concentrations of certain individual bile acids to those found in feces of rats fed low fat diets. Despite the significant effects of calcium on bile acid concentrations, minimal effects were observed on indices of colonic cell proliferation.