To examine the effects of exercise on plasma lipids and metabolism during lactation, sedentary, exclusively breast-feeding women were randomly assigned to an exercise (E) or control (C) group at 6–8 wk postpartum. E subjects performed aerobic exercise 45 min·d−1, 5 d·wk−1, for 12 wk. Resting metabolic rate (RMR), energy expenditure, body composition, and dietary intake were measured at 6–8, 12–14, and 18–20 wk postpartum. Maximum oxygen uptake (VO2max), postprandial insulin, glucose, and thermic response, and plasma lipid levels were measured at 6–8 and 18–20 wk. VO2maxincreased by 25% vs 5% in the E vs the C group, respectively (P< 0.0001). RMR was similar between groups and did not change over time. Weight and percent body fat declined (P< 0.01) during the study, but there was no difference between E and C groups. Exercise marginally-increased high-density lipoprotein cholesterol levels (P< 0.08), but did not affect other lipid concentrations. Insulin response decreased as VO2maxincreased (P= 0.05). There was no effect of time or group on glucose or thermic response. Exercise improves cardiovascular fitness during lactation, but does not increase the rate of postpartum weight loss.