Blood volume, plasma renin activity (PRA) and urine aldosterone excretion (UAE) were measured in ten very low birthweight infants who had a Grade 3 or 4 intraventricular haemorrhage (IVH) during the first 2 days after birth. Mean (range) birthweight was 950 (630-1500) g and gestational age was 27 (23-31) weeks. Nine infants were receiving assisted ventilation and one was breathing spontaneously. Eight IVH occurred on the first postnatal day and two on the second; seven were symptomatic and three asymptomatic. PRA was significantly higher than control values on Day 1 only; median 244 (range 91-654) ng/ml per h vs. 64 (4-259) ng/ml per h (P< 0.01). Infants with symptomatic IVH in the preceding 8 h (n = 6) all had PRA > 300 ng/ml per h; none of these infants had received transfusions or volume expansion between IVH and PRA measurement. PRA was < 100 ng/ml per h in the three infants with asymptomatic IVH and one infant with > 24 h interval between IVH and PRA measurement; three of these four had received transfusions prior to PRA measurement. UAE was not significantly different from control values on either Day 1 or Day 2. Blood volume at 22 ± 3 h postnatal age ranged from 75 to 107 ml/kg. There was an inverse logarithmic correlation between PRA and blood volume (r = 0.883; P < 0.005), with PRA values exceeding 300 ng/ml per h when blood volume was < 90 ml/kg. UAE did not correlate with either PRA or blood volume. These data suggest that very premature human infants are able to increase PRA in response to blood volume depletion. The levels of PRA observed do not suggest that the renin-angiotensin system of such preterm infants is less responsive to hypovolaemic stress than that of more mature infants. © 1989.