Little data exist regarding the acute maternal and fetal cardiovascular effects of caffeine ingestion. The purpose of this study is assess the cardiovascular effects of caffeine ingestion in pregnant subjects and their fetuses. We examined seven caffeine-naive gravidas longitudinally at 25.7 (SE +/- 0.7) and 36.1 (+/- 0.7) weeks' gestation, before and after ingestion of a caffeine citrate solution (100 mg/m-2 body surface area). Maternal pulse, blood pressure, fetal heart rate, and fetal heart rate accelerations were determined before and after caffeine ingestion. Uterine artery, fetal aorta, and umbilical artery flow velocity waveforms were examined at similar intervals. Significant before and after caffeine differences, regardless of gestational age, were noted for maternal pulse (85.1 vs 74.2 beats/min), diastolic blood pressure (65.9 vs 73.8 torr), mean arterial blood pressure (81.5 vs 87.5 torr), uterine artery systolic-to-diastolic (S/D) ratio (1.9 vs 2.4), fetal heart rate (144.6 vs 135.4 beats/min), and fetal heart rate accelerations (1.6 vs 4.2 per 30 minutes). Differences for fetal aortic and umbilical artery S/D ratios were not significant with respect to caffeine administration. Fetal aortic peak velocities increased with caffeine (85.5 vs 130.2 cm/sec) and the increase was augmented with advancing gestational age. We conclude that maternal ingestion of modest amounts of caffeine citrate in caffeine-naive subjects significantly affects both the fetal and maternal cardiovascular systems.