The extent of plasma protein binding of diazepam (DZ) and its major metabolite, desmethyldiazepam (DMDZ), was determined by equilibrium dialysis in plasma samples drawn from 62 nonfasting unheparinized volunteers aged 20 to 85 years. The free fraction for diazepam averaged 1.48 per cent (range 0.85 to 2.30 per cent) and increased with age (r = 0.33). Desmethyldiazepam also was extensively bound. The mean free fraction was 2.97 per cent (range 1.78 to 5.28 per cent) and increased with age (r = 0.27). Free fractions for both diazepam and desmethyldiazepam were negatively correlated with plasma albumin concentration (r = --0.17 and --0.39). However, age, sex, and albumin explained only a small proportion of variability in free fraction for either compound. Free fraction for desmethyldiazepam always exceeded that for diazepam, and the two were correlated (r = 0.32). Thus, at any given total plasma concentration, the unbound concentration of desmethyldiazepam will exceed that of diazepam.