Healthy term infants have higher umbilical cord GH levels compared to older infants and children. In the sheep, GH concentrations rapidly fall within an hour of birth; the physiology of GH release after parturition in the human term infant is less well known. The purpose of this study was to investigate spontaneous 12-h GH release in male and female term infants of varying postnatal ages. We studied 14 infants (7 males and 7 females). Subjects were divided into those studied earlier following delivery (28.2 +/- 3.4 h of age, mean +/- SE) and into those studied at a later time (74.8 +/- 3.5 h, P < 0.0005). The age at study was defined as the age (hours) when blood sampling began. There were eight infants studied at an early age (four males and four females) and six studied at a later age (three males and three females). Subjects were comparable with respect to gestational age, birth weight, and length; all were biochemically euthyroid. One infant was large for gestational age although his head circumference was in the normal range. Blood (0.1 mL) was taken every 30 min over a 12-h period from an indwelling umbilical catheter; no stress occurred during the blood withdrawal. GH was determined by a double-antibody RIA using 0.01 mL plasma. GH pulse detection was undertaken using Cluster, a computerized pulse detection algorithm. Total insulin-like growth factor I (IGF-I) was measured following separation of the IGFs from the serum binding proteins. Spontaneous pulsatile GH release was observed in all infants studied. No differences in GH characteristics were found between male and female subgroups in the early or late study groups. In subsequent analysis, the data for the males and females are combined. The GH pulse frequency per 12 h was greater in the earlier studied group, 5.1 +/- 0.9 (mean +/- SE) vs. 2.5 +/- 0.7 in the later group (P < 0.05). The maximal pulse amplitude was 47.1 +/- 7.9 micrograms/L in the early and 27.1 +/- 4.1 in the later studied group (P < 0.06). The incremental pulse amplitude was 26.4 +/- 3.4 micrograms/L in the early and 12.8 +/- 2.7 in the later group (P = 0.01). The pulse width was greater in the later studied group (202.8 +/- 71.1 min vs. 84.1 +/- 21.6, P < 0.06).(ABSTRACT TRUNCATED AT 400 WORDS)