Plasma glucose values have been reported to be lower in women than in men after a 72-hour fast. However, a comparison of glucose kinetics in fasting men and women has not been described. Therefore, five normal men and five normal women underwent sequential 3-3H-glucose infusions after both a 14- and a 64-hour fast. Plasma glucose levels fell similarly during the fast in men (5.23 +/- 0.03 v 3.96 +/- 0.14 mmol/L, P less than .01) and women (4.84 +/- 0.14 v 3.65 +/- 0.25 mmol/L, P less than .01). The fall in plasma glucose was associated with a significantly greater fall in glucose appearance (Ra) in women compared with men (P less than .05). Ra fell 15.8% +/- 3.0% in men (2.11 +/- 0.24 to 1.79 +/- 0.24 mg.kg-1.min-1, P less than .01) and 24.6% +/- 1.4% in women (2.22 +/- 0.17 to 1.67 +/- 0.12 mg.kg-1.min-1, P less than .001). During the fast, plasma glycerol, free fatty acids (FFA), and beta-hydroxybutyrate levels rose significantly and plasma alanine fell significantly in both sexes. Plasma glycerol levels were significantly higher in women compared with men after fasting (0.16 +/- 0.01 v 0.11 +/- 0.02 mmol/L, P less than .05). In addition, the transition from ambulation to bed rest demonstrated unexpected sex-related differences in glucose homeostasis after the 64-hour fast. During the two-hour equilibration period required for glucose kinetic studies (subjects reclining), significant decrements in glucose, FFA, and lactate were observed in the 64-hour fasted women but not in the men.(ABSTRACT TRUNCATED AT 250 WORDS)