The direct radioimmunoassay for plasma aldosterone (PA) set up in this laboratory has been reported elsewere. Subjects were 115 normal full term newborns and infants. During the first week of life, there was a wide dispersion in the PA levels (less than 1 - 445, n = 89), but on the whole these levels were high (greater than 6 ng/100 ml) and 35% were higher than those observed in children after stimulation (greater than 48 ng/100 ml). Furthermore, there was a negative correlation between PA values and the day of sampling (P less than 0.01). In older infants, PA values were lower and the ranges narrower. A highly significant negative correlation was found between Na:K ratios and PA levels. The mean geometrical value for PA levels was 53 ng/100 ml vs. 1.012 for Na:K mean ratio. When related to weight variations, PA values were randomly distributed. No correlation was found between PA and sodium intakes. The daily urinary excretion of tetrahydroaldosterone and 3-oxoconjugate of aldosterone were lower per se, but when related to surface area, were found to be roughly 3-fold those of control adult subjects. We conclude that in the newborn, both the production and excretion of aldosterone are different than values in the adult and that responses to the regulation factors mentioned above are decreased.