In seven healthy male volunteers the effects of the pattern of dosing on the pharmacokinetics of diazepam have been studied. A cross-over design was employed that consisted of three parts: a single intravenous dose (0.1 mg/kg), and oral dosing (10 mg/day) for six days followed by an intravenous bolus (0.1 mg/kg) on the seventh day, followed by re-examination of a single intravenous dose after diazepam (D) and its major metabolite desmethyldiazepam (DD) had been completely eliminated. Plasma levels of D and DD were monitored by a specific, sensitive GLC-method. In younger patients (n = 5, age 29 - 35 years) the elimination half-life, T1/2(beta), of D was 33.9 +/- 10.6 h (mean +/- S.D.) after the single dose. The control study gave an almost identical result (35.7 +/- 12.1). After subchronic dosage in all patients T1/2(beta) showed a modest but significant prolongation (paired t-test p less than 0.01) to 52.9 +/- 17.4 h. It was caused by a significant decrease (p = 0.016) in total plasma clearance (Cl), from 26.0 +/- 10.8 ml/min. Older patients (age 43-60 years) showed the same phenomenon. Blood/plasma ratios remained constant indicating no change in protein binding. Biliary excretion of D was measured in five patients with a T-tube. Only negligible amounts (0.3 - 0.4%) of administered D were excreted within 3 days after subchronic dosage, which demonstrates a lack of enterohepatic cycling of D. After multiple administration of D, there was accumulation of DD to levels approximately five times higher than after a single dose. The possibility that the slower elimination of D after subchronic treatment might be caused by DD was also supported by experiments in dogs and rabbits. After pretreating rabbits with DD and maintaining a high DD plasma level, there was prolongation of T1/2(beta) from 2.7 h to 5.2 h, with a corresponding decrease of Cl from 101.6 ml/min to 23.4 ml/min. Similar results were obtained in dogs. It is concluded that the disposition of D is altered by subchronic use and may be regulated by the plasma DD concentration.