The short time distribution of58Co in mice that received a58CoCl2solution intravenously or intraperitoneally was examined by means of scintillation measurements. The serum, kidney, pancreas, spleen, liver, and heart were analyzed. Statistically significant differences in the distribution pattern after intraperitoneal and intravenous administration were noted for serum, kidney, liver, and heart 1 h after injection and for kidney, liver, and heart 5 h after injection. Only the heart showed a significant difference between the two administration methods 24 h after injection. Intravenous injections gave less variation between animals of the58Co distribution pattern than intraperitoneal injections.