Three groups of mature rams were maintained on diets of hay, hay + 2% lupin or hay + 2% cowpea for 11 weeks. Serial blood samples were taken at 15-min intervals for 12 h for the determination of GH and IGF-I content by radioimmunoassay and for IGF-binding protein-3 (IGFBP-3) levels by Western blotting. The rams were killed after 77 days of supplmentary feeding and their pituitary glands analysed for content and GH and GH mRNA. Mean plasma GH and baseline GH levels were significantly (P < 0.01) decreased in the rams fed lupin and cowpea compared with controls fed hay and GH pulse amplitude was signifciantly (P < 0.001) decreased in the group fed the cowpea diet. The frequency of GH pulses was not significantly altered by either treatment. Plasma concentrations of IGF-I were elevated in rams fed lupin (P < 0.001) or cowpea (P < 0.05). IGFBP-3 levels were not significantly (P > 0.05) altered by either treatment. There were no significant differences in pituitary content of GH mRNA but pituitary content of GH was increased in rams fed lupin (P < 0.05) and cowpea (P = 0.07). In conclusion, a high-protein diet decreased plasma GH levels and increases IGF-I without changing plasma IGFBP-3 levels in rams. Thus ongoing synthesis of GH, as indicated by the mRNA levels, may cause a build up of GH stores in the pituitary gland.