This study examined the substrate and hormonal responses during exercise classes for 36 pregnant women at selected stages of pregnancy. Six nonpregnant women served as controls. Blood samples were obtained before exercise, at the end of aerobic exercise, and 45 min after aerobic exercise. All samples were analyzed for substrates (glucose, lactate, FFA) and 7 hormones. Heart rates in the pregnant women were approximately 150 bpm, although some were as high as 180 bpm. Heart rates in the control group were approximately 130 bpm. Modest increments occurred in lactate and FFA during exercise, and decrements in glucose were observed in all groups. Insulin concentrations decreased with exercise and remained depressed after exercise whereas FFA remained elevated postexercise in the pregnant women. Modest transient changes were observed with exercise in the other hormones. A severely blunted norepinephrine response to exercise was observed in the third-trimester group. In conclusion, uncontrolled non-steady-state exercise does not disrupt the substrate and endocrine milieu markedly. Recovery in hormonal and substrate concentrations appears to occur rapidly after exercise. However, decrements in blood glucose do occur for a short time during prenatal exercise classes, most notably for women in the third trimester.