Distribution and excretion of the radioactivity of cyclocarbothiamine-^<35>S injected intravenously into mice were studied by means of whole body autoradiography, microautoradiography and liquid scintillation spectrometry. The whole body autoradiography showed that the radioactivity was elicited in almost all organs by 3 minutes after the intravenous administration. It was especially prominent in urine, liver salivary gland, kidney, small intestine, pancreas and Harderian gland behind eyeball. Thiamine-^<35>S injected intravenously gave rise to radioactivity in salivary gland, Harderian gland, skeletal muscle and blood in heart, but it was lower than that of cyclocarbothiamine-^<35>S. By 6 hours, the activity became noticeable in the content of the large intestine in both cases. Microautoradiography showed higher activity in the muscular layer of small intestine of mice receiving cyclocarbothiamine, whereas in that of thiamine administered mice, it was diffusely distributed throughout the mucosal, submucosal muscular layers. The result obtained by scintillation counting was consistent with that by autoradiography. Higher level of ^<35>S was detected in muscle, salivary gland, lung and heart of cyclocarbothiamine-injected mice than in those of thiamine-injected ones. The blood level kept on higher in the former, and the excretion into urine was more rapid in the latter.