The role of GH-releasing factor (GRF) in regulating somatic growth was studied in rats by passively immunizing animals with antisera raised against rat GRF. GRF antiserum (GRF-ab) treatment significantly inhibited the normal increase in body weight observed in both young male and female rats. Similar studies conducted in neonatal rats (days 1-23 of age) also demonstrated that GRF is critical in regulating neonatal growth. The biological half-life of the GRF-ab was determined to be between 2.5 and 3.5 days in these animals. The effects of GRF-ab and somatostatin antiserum (SRIF-ab) treatment on serum GH concentrations were also studied in neonatal rats. Between 1 and 20 days of age, GRF-ab treatment significantly decreased serum GH concentrations compared to those in control animals. From days 1-10 of age, SRIF-ab treatment had no effect on GH concentrations, whereas on days 15 and 20 of age, SRIF-ab treatment significantly increased GH concentrations. The results demonstrate that hypothalamic GRF is critical in regulating serum GH concentrations and somatic growth in neonatal and young animals. They also suggest that the role of SRIF in regulating GH concentrations is not established until sometime after 10 days of age.