The absorption of proscillaridin A was studied in four patients undergoing catheterization of the portal vein for diagnostic purposes. Proscillaridin 1.5 mg was given as a single oral dose and plasma glycoside activity was analyzed by the 86Rb-uptake inhibition technique. Proscillaridin appeared rapidly in the portal blood, peak activity being found after 15 min in three and after 30 min in one patient. In peripheral blood the peak activity occurred after approximately 35 min. Despite rapid passage across the gut wall, porto-peripheral differences in glycoside activity were small; they were zero after 4h. The mean amount absorbed as active porscillaridin during the first 4h after the dose was calculated to be only 7.1% of the given amount. Late porto-peripheral differences, probably due to enterohepatic recycling, appeared after 6h in three patients; The results suggest that proscillaridin undergoes first pass inactivation in the gut wall. Enterohepatic recirculation may contribute to the amounts of active glycoside that reach the systemic circulation.