Eight healthy men (17–31 yr) were studied in the postabsorptive state(control) and after a 59-h fast. They had venous blood drawn at rest and during 10 min of moderate work (treadmill running requiring 60% of the individual maximal oxygen uptake) succeeded by 5 min of heavy work (requiring 100% of the individual maximal oxygen uptake). Fasting resulted in a halving of insulin concentrations, a slight reduction in glucose concentrations, and ketonuria. Comparing levels during heavy work between 59–h–fasted and postabsorptive subjects, the concentrations of norepinephrine [36.5 ± 3.7 us.19.5± 2.5 nmol liter−1 (mean and SE)], epinephrine (6.8 ± 2.1 us.2.0 ± 0.4 nmol liter−1),glucagon (18.9 ± 2.9 us. 7.5 ± 0.7 pmol liter−1), GH (77 ±11 us.34 ± 14 µU ml−1), cortisol (491 ± 97 us. 34 ± 44 nmol liter−1), PRL (19 ± 5 us. 11 ± 2 µg liter−1), FFA, glycerol, and lactate were higher after fasting than in control experiments. The concentration of vasoactive intestinal polypeptide was higher at rest, but not significantly so during exercise. Hematocrit, heart rate, arterial blood pressure, and sodium and potassium concentrations were not significantly changed by fasting. When insulin levels in fasting experiments were increased above levels in control experiments by the infusion of glucose during exercise, the concentrations of counterregulatory hormones in plasma, except for PRL, were still higher than those in control experiments. It is concluded that during exercise, men who have fasted for 59 h, like other hypoinsulinemic persons such as fatfed persons and poorly controlled diabetics, have higher concentrations of counterregulatory hormones in plasma than normoinsulinemicmen. It is suggested that insulin availability before, but not during, exercise is an important determinant of the hormonal response to exercise.