Strenuous muscular exercise is known to induce increases in plasma values of adrenalin (A) and noradrenaline (NA) and a twofold increase in plasma renin activity (PRA) in healthy men and women. The effect in pregnant women is not known. In 36 pregnant women, systolic blood pressure and plasma levels of PRA, A and NA were determined at rest, during an orthostatic test and during submaximal leg exercise on a bicycle ergometer. Eight of these women were re-examined after delivery and served as a non-pregnant control group. The pregnant women had a significantly greater increase in systolic blood pressure than the non-pregnant subjects. PRA showed a sevenfold increase during pregnancy, but exercise caused no further change in plasma values. As expected, the non-pregnant PRA values doubled during exercise. Resting values of A and NA were not altered by pregnancy. Near-maximal exercise caused a significant rise in A and NA, both in pregnant and in non-pregnant women. The results indicate that there is no activation of the sympathetic nervous system either at rest or during strenuous exercise in pregnancy and that the sensitivity of the circulatory system seems to be well adjusted to the increased activity of the renin angiotensin system during pregnancy.