Systemic blood pressure (measured with a zero-randomized sphygmomanometer), stroke volume and heart-rate (measured with a Minnesota Impedance Cardiograph), hematocrit, and their derivatives -- cardiac output and peripheral vascular resistance--have been assessed in three groups of subjects. First, a control group of 19 nonpregnant women were matched for age and weight with the subjects in the second group, which consisted of 19 patients who were seen at regular intervals on 12 to 15 occasions from 8 to 11 wk of pregnancy until 6 wk postpartum. The third group consisted of 8 patients seen from before conception, throughout pregnancy and to several months postpartum. Readings were made with the subject in each of six positions: supine, reclining, left and right lateral, left and right tilt. This paper concerns the readings obtained in the left lateral position. The data showed that pulse rate rose throughout pregnancy. Stroke volume and cardiac output rose shortly after conception, the increase over the prepregnancy level being statistically significant by 12 wk. Thereafter both values fell throughout the rest of pregnancy and were below prepregnancy levels by about term, taking some weeks to regain the prepregnancy value. There were irregular fluctuations in the level of systolic blood pressure; diastolic blood pressure fell during the first 16 wk and then rose to reach almost the prepregnancy value by term. Peripheral resistance fell during the first trimester, then increased markedly throughout the remainder of pregnancy.