Four groups of male rats were given the following oral treatment: control group (n = 20) deionized drinking water, Mn group (n = 20) deionized drinking water containing 56 ppm Mn2+ (1 mmol/l), Cd group (n = 10) deionized drinking water containing 112 ppm Cd2+ (1 mmol/l) and Cd+Mn group (n = 10) deionized drinking water containing 112 ppm Cd2+ and 56 ppm Mn2+. Half of each group was sacrificed after 4 weeks and the other half after 8 weeks of treatment. At each time interval, the mean levels of Mn in blood, in urine and in the various tissues did not differ between the control and Mn groups. Furthermore, comparable Mn levels were found after 4 and 8 weeks of treatment. Microscopical examination of the brain failed to reveal any ovr!rt morphological alteration in the Mn group. With respect to the control group, the Cd and Cd+Mn groups exhibited increased levels of Cd in blood, urine, liver, whole kidney, kidney cortex and in brain (cortex, cerebellum, basal ganglia), but the Cd + Mn groups showed invariably lower levels than the Cd group after 4 weeks as well as after 8 weeks. These results suggest that the rate of gastrointestinal absorption of Cd is decreased by supplementation of the drinking water with a 'non-toxic' dose of Mn2+.