Basal plasma GH levels and the GH responses to an injection of 1 microgram/kg 1-44(NH2) GHRH were determined on day 3 postnatally in 5 small gestational age (SGA) twin newborns and their appropriate gestational age (AGA) co-twins, and in 10 SGA singleton newborns and 6 AGA singleton newborns. The mean basal plasma GH level was higher in the SGA than in the AGA infants but the difference was significant only for singleton newborns (p less than 0.01). The mean peak plasma GH level was markedly increased in SGA compared to AGA infants (p less than 0.05 for twins, p less than 0.01 for singletons). Twelve SGA infants re-tested at 1 month had lower basal and peak plasma GH levels (p less than 0.001 and p less than 0.01). In 21 SGA and 17 AGA infants, serum IGF-I, measured by RIA between 12 and 96 hours after birth, was significantly higher in SGA than in AGA (p less than 0.001). These results suggest that, whatever the mechanism, functional hypersomatotropism is present at day 3 in SGA infants. This hypersomatotropism may participate in the early catch-up growth process.