The effect of the synthetic human pancreatic GH releasing factors (hpGRFs, hpGRF44 and hpGRF40) on GH release was studied both in vivo and in vitro. Four-week-old cockerels were injected iv with hpGRFs at 0.1 microgram, 1.0 microgram, and 10.0 micrograms/bird. The 0.1 microgram dose of hpGRFs had no effect on plasma GH levels, but the 1.0 microgram and 10.0 micrograms doses of hpGRFs caused a dose-dependent elevation of plasma GH. Plasma GH levels returned to basal 60 min after injection. HpGRFs, thyrotropin releasing hormone (TRH) and somatostatin (SRIF) were examined in a chicken in vitro pituitary cell culture system. In vitro hpGRFs and TRH caused a significant dose-dependent release of chicken GH, and the ability to stimulate GH was additive when hpGRF44 and TRH were added together in the culture medium. SRIF showed no consistent effect on the release of chicken GH, but it inhibited the stimulatory effect of hpGRF44 on chicken GH release. We conclude that hpGRF is a potent releaser of chicken GH in vivo and in vitro.