Ten normal men and 9 normal women were fasted for 72 hr. Four men, matching 4 of the women in weight and age, were also fasted a second time while receiving ethinylestradiol (0.1 mg twice daily). Five of the women were fasted in both the preovulatory and postovulatory phases of the menstrual cycle. Premenopausal women had significantly lower concentrations of glucose during fasting than men. The mean lowest glucose concentration in plasma in the male group during the 3 days of starvation was 66.4 ± 2.9 mg/100 ml (X̄ ± sem). The corresponding figure in females was 47.8 ± 2.9 mg/100 ml (p < 0.01). Associated with the greater decrease of plasma glucose in women was a significantly greater and earlier increase of plasma immunoreactive glucagon. Mean peak glucagon concentrations in women and men were, respectively, 230.0 ± 36.1 μμg/ml and 121.8 ± 17.9 μμg/ml (p < 0.01). These differences of glucagon were, likewise, significant when expressed in terms of per cent increase from basal (170 ± 25% vs. 56 ± 20%, P < 0.01). There was no significant difference in plasma insulin concentrations during the fast, but plasma free fatty acid (FFA) concentration rose significantly higher in the women than in the men (mean peak concentration: women, 2.25 ± 0.204 mmole/1 vs men, 1.429 ± 0.149 mmole/1, p < 0.01). An attempt to correlate the hormone-substrate response with (a) body weight of the subjects, (b) treatment or nontreatment with estrogen or (c) phases of the menstrual cycle was unsuccessful. The data obtained from these fasts supports a different hormone-fuel pattern of response for men and women. The data are not consistent with a single, primary hormone of fasting.