We have studied the rebound secretion of GH following short-term somatostatin (SS) infusions in conscious rats, using an automatic sampling system for withdrawing frequent microsamples of blood. Intravenous infusions of SS (5–50 μg/h per rat) inhibited spontaneous GH secretion, but when SS was withdrawn there was a large burst of rebound GH secretion. A sub-anaesthetic dose of urethane reduced such rebound bursts of GH, suggesting a hypothalamic involvement in rebound GH secretion. Passive immunization with an antibody against rat GH-releasing factor (GRF) attenuated the rebound GH secretory response to the withdrawal of an SS infusion (GH concentration during rebound secretion was 26±21 μg/l vs 475 ± 127 μg/l (mean ± s.e.m.), after 0·5 ml anti-GRF serum or non-immune serum respectively). The inhibition of GH rebound secretion was related to the dose of anti-GRF serum administered. Intravenous infusions of human GH (20– 100 μg/h per rat) also reduced the size of the rebound GH secretion following SS withdrawal, in both male and female rats. We suggest that the rebound GH secretion that follows SS withdrawal in vivo is caused mainly by a hypothalamic release of GRF. Exogenous GH inhibits SS-induced rebound GH secretion in the conscious rat, possibly by inhibiting hypothalamic GRF release.