Observations of a variable anticoagulatory response in patients being treated by phenprocoumon and wheat bran resulted in a study of seven healthy volunteers. Up to 168 h after the administration of phenprocoumon, both in the fasting state and following the ingestion of 35 g wheat bran in a crossover design, blood samples were taken and urine was collected. The usual pharmacokinetic parameters were calculated from the concentrations measured. Following the ingestion of wheat bran, a decreased absorption rate for phenprocoumon but no decrease in overall bioavailability was observed. In addition, a decrease in total body clearance of phenprocoumon and an increase in the free plasma fraction of phenprocoumon, resulting in decreased free drug clearance, was seen after wheat bran administration. No differences were observed in the urinary excretion of either phenprocoumon or its metabolite. These findings cannot be completely explained by our present knowledge and need further investigation.