1. Glucose metabolism and changes in the concentrations of several hormones in jugular plasma were measured in growing lambs fed on fresh pasture ad lib. One group of lambs acted as control while the second received a continuous abomasal infusion supplying 44 g sodium caseinate + 0·5 g L-methionine/d.
2. Hormone concentrations were determined by radioimmunoassay procedures and glucose irreversible loss measured from continuous infusion of D-[U–14C]glucose.
3. Protein infusion increased plasma concentrations of insulin, glucagon and thyroxine (T4), depressed those of growth hormone, prolactin and somatostatin and had no effect on triiodothyronine (T3) concentrations. Cortisol concentrations also tended to be slightly higher in the plasma of protein-infused lambs.
4. Increases in herbage intake within the ad lib. range were associated with increases in plasma insulin and glucagon concentrations and decreases in growth hormone concentration, and it is suggested that these effects could be mediated in part by the accompanying increases in protein absorption from the intestines. The T4:T3 value also decreased with increasing herbage intake, and it is suggested this was due to conversion of T4 to T3.
5. After correction by covariance to equal herbage intake, rates of irreversible glucose loss for control and protein-infused lambs were 9·2 and 10·0 mg/min per kg body-weight0·75. It was calculated that respectively 0·12 and 0·19 of the total glucose production in control and protein-infused lambs could be accounted for by net synthesis from protein.
6. It was concluded that changes in the circulating concentration of several hormones in protein-infused compared with control lambs were likely to have been implicated in protein deposition forming a greater proportion of energy retention in the infused lambs (0·41 v. 0·27).