Whole body retention and tissue distribution of 58Co in mice having had an oral cavity depot of 58CoCl2 was studied at 1, 5 and 24 h after administration by means of gamma scintillation measurements and autoradiography. The whole body 58Co retention at 5 h after administration was 93.3% of the whole body 58Co content at 1 h. Corresponding 58Co-retention at 24 h after administration was 9.2%. Tissues analyzed by gamma scintillation measurements at 1, 5 and 24 h post administration were: serum, kidney, pancreas, spleen, liver, heart, lung, tongue and palatal mucosa. At all survival times after administration palatal mucosa showed the highest 58Co median content followed by tongue. The very high 58Co median content of palatal mucosa and tongue at 1 h after administration showed a strongly decrease at 5 and 24 h after administration. For the other tissues the highest 58Co median content was found in the liver and the lowest in the spleen with the highest 58Co median value for each tissue at 5 h after administration. The autoradiograms revealed a high 58Co uptake in oral, nasal, pharyngeal and esophageal mucosa, intestines and stomach. In liver, pancreas and kidney no uptake was observed at 1 h and a weak uptake was detected at 5 and 24 h.