After single intracaecal administration of [14C]rhein (25 mg kg-1) and [14C]rhein anthrone (20 mg kg-1) to rats, the summated recovery rates of 14C after five days were in urine 37(+/- 8.3)% and 2.8(+/- 0.4)% and in faeces 53(+/- 9.5)% and 95 (+/- 10.1)%, respectively. The clearance of radioactivity from the organs and tissues was almost complete within three days, with the exception of the kidney which exhibited pronounced retention of radioactivity even after five days (less than 61% of 24 h values). Extracts of faeces from animals treated with [14C]rhein of [14C]rhein anthrone, revealed rhein as well as other radioactive substances, which chemically did not react as 1,8-dihydroxyanthraquinones.