Fourteen women performed treadmill exercise for continuous 10-minute periods at levels of 2, 3 and 4 mets in the second and third trimester of pregnancy and again postpartum. The percentage increase in heart rate in response to exercise was similar during pregnancy and postpartum while systolic blood pressure was increased only during pregnancy. Maternal plasma noradrenaline levels at peak exercise increased 64% (p less than 0.005), 42% (p less than 0.005) and 29% (NS) in the 3 studies respectively compared to resting levels at these times. Ten women experienced increased uterine activity during the exercise or recovery periods. Fetal heart rate was increased (p less than 0.05) by exercise but this was independent of uterine contractions and plasma noradrenaline levels.