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Simultaneous measurement of plasma renin activity (PRA), plasma aldosterone concentration (PA) and urinary aldosterone excretion (UAE) was made using the RIA method along with determination of Na and K balance in 1-week-old neonates with gestational age of 30–41 weeks (mean 35.9 weeks) and birth weight of 1,160–4,670 g (mean 2,680 g). It was demonstrated that PRA decreased from the value of 36.3 ± 6.3 ng/ml/h (mean ± SE) to a level of 10.2 ± 2.1 ng/ml/h (p < 0.001), PA did not change and UAE increased from 3.3 ± 0.8 to 7.8 ± 1.4 μg/day (p < 0.01) as the gestational age advanced from 30–32 to 39–41 weeks. There was no correlation between either PRA and PA or PA and UAE. PRA showed a significant positive correlation with urinary Na excretion (p < 0.001) and plasma K concentration (p < 0.05), but it was negatively related to Na balance (p < 0.001). Significant negative correlations were found between UAE and urinary Na excretion (p < 0.05), urinary Na/K ratio (p < 0.01) and plasma K concentration (p < 0.05); however, UAE positively correlated with Na balance (p < 0.01). It is concluded that, in response to renal salt wasting and to the subsequent negative salt balance, premature infants can augment their PRA above values found for full-term infants. Their adrenals, however, failed to respond adequately to this stimulation.

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