The concentrations of the vasoactive substance, 5-hydroxytryptamine (5-HT), were estimated in whole blood samples from 12 pregnant women with hypertension in the third trimester. These were compared with a control group of 15 normotensive pregnant women of the same gestation. The hypertensive patients were admitted for observation and the whole blood 5-HT estimated again after four days bed rest: in all patients the blood pressure had, by this time, returned to within the normal range. No significant difference was found between the mean blood 5-HT concentrations of the hypertensive and the normotensive groups. A small, but not statistically significant, rise occurred in the hypertensive group after four days bed rest. Thus this study gave no evidence that 5-HT is implicated in hypertension in pregnancy.