The distribution of a single oral dose of 14C bithionol sulfoxide in mice was determined by whole body autoradiography. The bithionol sulfoxide was found in the whole organism mainly in richly vascularized organs, liver, kidney, lung. It did not pass through the blood-brain barrier and did not accumulate in any particular organ but persisted in the blood. Observations were performed in 18-day pregnant mice in order to investigate an eventual placental transfer. Six hours after the administration, radioactivity was detected in blood and fetal vascularized tissues.