These studies characterized the secretion of GH-releasing factor (GRF) and somatostatin (SRIF) into the hypophysial portal circulation in ewes after long term restricted feeding. In addition, we examined the temporal relationship between the concentrations of these two hypothalamic peptides in portal blood and the concentration of GH in jugular blood. Six sheep were fed 1000 g hay/day (normal feeding) and 6 sheep were fed 400-600 g hay/day (restricted feeding). This resulted inawt loss of 35% in restricted animals compared with 6% in control animals after 20 weeks. Fluctuations in portal levels of GRF indicated a pulsatile pattern of secretion with approximately 60% of pulses coincident with, or immediately preceding, a GH pulse. Similarly, 65% of GH pulses were associated with GRF pulses. Restricted feeding increased (P < 0.01) mean (± SEM) plasma GH levels (9.8 ± 1.4 vs. 2.9 ± 0.6 ng/ml) and mean GH pulse amplitude (7.9 ± 1.8 vs. 2.8 ± 0.3 ng/ml) but did not affect mean GH pulse frequency (6.0 ± 1.1 vs. 5.7 ± 1.1 pulses/ 8 h). The level of feeding had no effect on mean portal concentration of GRF (restricted: 5.5 ± 0.8, normal: 6.6 ± 1.4 pg/ml), GRF pulse amplitude (14.7 ± 2.3 vs. 13.5 ± 0.7 pg/ml), or GRF pulse frequency (5.3 ± 1.1 vs. 6.7 ± 0.9 pulses/8 h). Portal concentrations of SRIF in sheep on a restricted diet were half (P < 0.01) those of sheep fed a normal diet (10.2 ± 2.3 vs. 19.6 ±1.6 pg/ml). Pulses of SRIF were not significantly associated with changes in GH or GRF concentrations. These data indicate a functional role for hypothalamic GRF in initiating GH pulses. Furthermore, the increase in GH secretion in underfed sheep was most probably due to a decrease in the release of SRIF into hypophysial portal blood. Restricted feeding had no affect on GRF secretion, but because of the reduced exposure of the pituitary gland to SRIF, it is possible that responsiveness to GRF is enhanced.