The extent of intersubject variation in diazepam free fraction was measured in fasting plasma of 74 unrelated subjects. Free fraction differences between subjects were significant and ranged from 0.97% to 1.99%. Diazepam free fraction in 29 males was normally distributed about a mean of 1.25% (range, 1.05% to 1.47%), but the distribution in females was skewed to higher free fractions and 40% had values above the highest in males. Albumin concentration (r = -0.27, p less than 0.002) and age (r = 0.44, p less than 0.001) only accounted for a small part of the variation. Within-pair variances were not greater in 11 dizygotic than in 18 monozygotic twin pairs, indicating a greater contribution of environmental than of genetic factors to diazepam binding. The prehemodialysis free fractions of diazepam in 9 uremic patients ranged from 3.44% to 6.69%, and decreased (p less than 0.005) in 7 after 6 hr of hemodialysis. In 10 subjects determination of intrasubject variation in diazepam free fraction between 14-hr fasting and 2-hr postprandial plasma samples indicated that because subjects differ in their pattern of change in free fraction (p less than 0.001), the overall decrease in mean free fraction did not achieve statistical significance (p = 0.10). The mean relative percent change in free fraction within subjects after feeding was 15.2%.