1 Factors influencing steady-state plasma concentrations of diazepam (DZ) and its major metabolite desmethyldiazepam (DMDZ) were assessed in 110 male Veterans Administration outpatient clinic patients (mean age 53 years). 2 Patients reportedly had taken DZ for 1 to 14 years (mean duration 5.1 years) at a mean daily dose of 20 mg (range 2 to 55 mg). 3 Steady-state plasma concentrations of DZ (mean 329 ng/ml) and DMDZ (mean 389 ng/ml) were highly correlated (r = 0.80), with a mean DMDZ/DZ ratio of 1.26. 4 Weight-corrected daily dose were significantly correlated with plasma level of DZ (r = 0.32), DMDZ (r = 0.38) and the sum of DZ plus DMDZ (r = 0.37), but explained a small fraction of individual variation. 5 Duration of therapy, smoking habits, alcohol consumption, and number of other drugs coingested were not significantly related to plasma level.