The study was longitudinal in design. Measurements were made of glomerular filtration rate, renal plasma flow or uterine blood flow. To define first trimester changes, studies were conducted on in vitro fertilization and ovarian failure patients who had a broad spectrum of plasma prorenins. Since interrelated changes in renin and prostaglandins may participate in the pathogenesis of hypertensive pregnancy, RAAS, prostaglandins, and uteroplacental flow velocity profiles were monitored during a placebo-controlled trial of low-dose aspirin in chronic hypertensives.