Selected substrate and hormonal responses to exercise were compared in two phases of the menstrual cycle. Exercise-induced changes in substrate [glucose, lactate, free fatty acids (FFA), glycerol] and hormonal patterns [luteinizing hormone (LH), follicle-stimulating hormone (FSH), insulin, progesterone (P), growth hormone (GH), cortisol] were compared in the follicular and luteal phases of the menstrual cycle in 24-h-fasted (n = 5), glucose-loaded (n = 6; 1.50 g/kg, 20% solution), and control subjects (n = 8). A treadmill walk was maintained for 60 min (30 min, 40% VO2 max; 30 min, 80% VO2 max). Blood samples were obtained 5 min before, 15, 30, 45, and 60 min during, and 30 min after exercise. In the glucose group a blood sample was also taken 20 min before exercise, and glucose was ingested 15 min before exercise. Within each nutritional group the metabolic and endocrine responses to exercise were similar in the two phases for glucose, lactate, glycerol, LH, FSH, and cortisol (P greater than 0.05). In the glucose group the FFA response was lower in the luteal phase (P less than 0.05). In the fasted subjects insulin and GH responses were elevated in the luteal phase (P less than 0.05). P responses in the control and glucose groups were markedly greater in the luteal phase (P less than 0.05). In the fasted subjects no alteration in P occurred in either phase (P less than 0.05), and the LH concentration was lower in these subjects relative to the control groups (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)