Pregnancy is associated with marked changes in renal functions. Hypertensive disorders in pregnancy are frequently accompanied by deteriorated renal functions and by pathological lesions in the glomeruli. Using a pulsed Doppler ultrasound, we measured the resistance to flow in the renal artery in normal and hypertensive pregnant patients. We performed 176 pulsed Doppler ultrasound measurements of the renal arteries at various stages of gestation in 60 low risk pregnant patients. Another 111 Doppler measurements of the same vessels were obtained from 80 hypertensive pregnant patients throughout the third trimester. Twenty-six patients had preeclampsia, 35 patients had pregnancy-induced hypertension, and 19 patients had chronic hypertension. The resistance to flow in the renal arteries (expressed by the resistance index, RI) did not change significantly during pregnancy. The renal artery RI was not significantly different between the hypertensive group (RI = 0.61 +/- 0.06) and the low risk group (RI = 0.605 +/- 0.04). The RI values were also similar in all hypertensive patient categories: preeclampsia (RI = 0.62 +/- 0.07), pregnancy-induced hypertension (RI = 0.60 +/- 0.06), and chronic hypertension (RI = 0.61 +/- 0.05). The renal artery RI did not correlate with the severity of the hypertensive disease or with the status of renal functions. The outcome of pregnancy was worse in patients with abnormally elevated resistance index in the renal arteries. Patients with abnormally elevated resistance to flow in the uterine arteries also had a significant increase in the resistance index in the renal vessels.(ABSTRACT TRUNCATED AT 250 WORDS)