The purpose of this study was to determine the acute effects of albumin infusion on blood volume and renal function in preterm infants with RDS and low total serum protein values. Ten infants (gestational age 28 to 36 weeks, body weight 0.88 to 2.46 kg) were given albumin 1 gm/kg (as 25% iv solution) over a ten-minute period. Within ten minutes after infusion was completed, total serum protein concentration, colloid osmotic pressure, and blood volume rose significantly while hematocrit fell from their preinfusion levels (P < 0.0005). Mean arterial blood pressure showed a smaller and less clear-cut increase (P < 0.05). Creatinine clearance rose significantly with infusion; even though preinfusion clearances correlated poorly with gestational age (r = 0.43), postinfusion clearances correlated well (r = 0.92). No significant rises in urinary flow rate Uosm/Posm, or free-water clearance were observed. These results indicate that albumin infusion acutely increases both blood volume and glomerular filtration in premature infants with RDS.