ABSTRACT. Serum luteinizing hormone-human chorionic gonadotropin bioactivity (B-LH) was measured daily in seven male and four female full-term newborns during the first 7 days of life. The B-LH levels were elevated in both sexes during the 1st day of life; subsequently, values decreased in both sexes. In males, they reached a nadir on the 4th day of life. A gradual secondary rise was then observed with B-LH levels on the 7th day significantly higher than on day 4 ( p < 0.025). By contrast, the B-LH levels in the females continued a gradual decline to levels significantly lower on day 7 as compared to day 4 ( p c 0.05). To determine whether pulsatile B-LH secretion occurs in newborns, serum concentrations were measured every 20 min for 2 h in eight male and seven female fullterm neonates on the 7th day of life. Pulsatile secretion of B-LH was detected in six males and six females. This study demonstrates that pulsatility of gonadotropin secretion is characteristic of neonates as early as 7 days of life and that there is a dichotomy between the levels of B-LH in males and females; levels in females decline progressively from day 1 through 7, whereas in males, a nadir is reached on day 4 with a secondary rise developing thereafter. This male sex-specific rise is presumably the drive responsible for the characteristic postnatal increase in testosterone which peaks at 1 to 2 months of age. (Pediatr Res 23: 530-533,1988) Abbreviations LH, luteinizing hormone hCG, human chorionic gonadotropin B-LH, bioactive luteinizing hormone RICT assay, rat interstitial cell testosterone assay 2nd-IRP, second international reference preparation hMG, human menopausal gonadotropin FSH, follicle-stimulating hormone LHRH, luteinizing hormone-releasing hormone so that by the end of the 1st wk the levels have diminished to a nadir, at which they are comparable to the levels characteristic of normal childhood. A second and longer sustained rise of plasma testosterone commences in the 2nd wk of life. Testosterone concentrations rise progressively to plateau during the 2nd month of life at levels that are not seen again until puberty (2, 3). This biphasic course of plasma testosterone concentration presumably relates to changes in testicular secretion and has been correlated with transient elevations in plasma LH concentrations as measured by immunoassay (4).The present study was undertaken to describe the changes in plasma gonadotropins of the LH-hCG class as measured by bioassay during the first 7 days of life. It was also designed to show whether newborns display pulsatility of gonadotropin secretion of the type seen in normal puberty (5). Finally, it afforded an opportunity to contrast the pattern of bioactive gonadotropins in the male with that in female neonates. MATERIALS AND METHODSClinical studies. Studies were carried out in two groups of neonates according to protocols approved by the Human Studies Committee of the Universidad del Valle. The first group consisted of seven male and four female newborns. All of the infants were ...