In a prospective manner, sequential plasma renin activity (PRA) was measured in a group of sixteen non-sick premature infants on day 1, 3, 10, 17 and 24 of life. These infants had the following criteria in order to enter the study: breastfeeding, Apgar score greater than 7 at 5 minutes and no medical problems. PRA reached the maximum level by day 10,139 ng/mL/h declining slowly over the next two weeks, but remaining higher than on day one of life. Serum sodium levels were also measured since day one of life; these levels were found low during the entire time of study. We can speculate that our higher PRA values could be related to the physiologic hyponatremia found more accentuated between the third and fourth weeks of life.