The biotransformation pathways following oral administration of prazepam are characterized. The time course of the active metabolite descyclopropylmethylprazepam (= norprazepam), representing 80-90% of all prazepam metabolites in plasma, can be described by an open one compartment model with a mean elimination half life of 70 h and a plasma clearance of 1.0-1.4 l/h. 3-5 h after single doses of 20 mg prazepam, the corresponding peak levels of norprazepam are 120-160 ng/ml. There is only a slight uptake of norprazepam into red blood cells and a relative low secretion into breast milk. Following repeated doses of daily 20 mg prazepam steady state levels of norprazepam are 600-800 ng/ml. The slow build-up of the active metabolite and the long elimination half life ensure continuous plasma levels with small fluctuations.