To determine whether vigorous exercise affects lactation performance, we compared well-nourished exercising (n = 8) and sedentary (n = 8) women whose infants were 9-24 wk old and exclusively breast-fed. Measurements included resting metabolic rate (RMR); maximum oxygen consumption (VO2max); plasma prolactin, cortisol, insulin, and T3; and body composition. Each subject completed a 3-d record of dietary intake, physical activity, and milk volume (by test weighing) and collected 24-h milk samples. Exercising women differed significantly from control subjects in VO2max (46.4 vs 30.3 mL.kg-1.min-1), percent body fat (21.7 vs 27.9%), total energy expenditure (3169 vs 2398 kcal/d), and energy intake (2739 vs 2051 kcal/d). There was no difference between the groups in plasma hormones or milk energy, lipid, protein, or lactose content. Exercising subjects tended to have higher milk volume (839 vs 776 g/d) and energy output in milk (538 vs 494 kcal/d). Thus, there was no apparent adverse effect of vigorous exercise on lactation performance.