The effects of age, sex, and obesity on the kinetics of single intravenous (iv) and oral doses of midazolam were evaluated in healthy volunteers who received 2.5-5 mg of iv midazolam on one occasion and 5-10 mg orally on another. Kinetics were determined from multiple plasma midazolam concentrations measured during 24 h after dosage. Midazolam elimination half-life (t1/2) after iv dosage was significantly prolonged in elderly (aged 60-74 yr) versus young (24-33 yr) males (5.6 vs. 2.1 hours, P less than 0.01) and total clearance was significantly reduced (4.4 vs. 7.8 ml X min-1 X kg-1, P less than 0.01), leading to increased systemic availability of the oral dose (50% vs. 41%, P less than 0.05). However total volume of distribution calculated by the area method (Vd) (1.6 vs. 1.3 1/kg) and protein binding (3.5 vs. 3.4% unbound) did not differ between groups. Among women there were no significant differences between elderly (64-79 yr) and young (23-37 yr) volunteers in t1/2 (4.0 vs. 2.6 h), clearance (7.5 vs. 9.4 ml X min-1 X kg-1), Vd (2.1 vs. 2.0 1/kg), protein binding (3.7% vs. 3.7% unbound), or oral bioavailability (38% vs. 36%). In obese volunteers (mean weight 117 kg; 173% of ideal weight) versus control subjects of normal weight (66 kg, 95% of ideal weight) matched for age, sex, and smoking habits, midazolam Vd was increased significantly (311 vs. 114 1, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)