When butyric acid was injected intravenously at an amount of 2.5 mM/kg, blood glucose rose markedly in normal sheep. In a ewe with pregnancy toxemia blood glucose did not change. When the same amount was injected intraruminally to sheep and cows, the plasma level of non-esterified fatty acids, NEFA, was in most cases decreased by about 30—50 %, while blood glucose remained almost unchanged. The same results were obtained when the double amount, 5 mM/kg, was used. The depressant effect of butyric acid on NEFA after intravenous and intraruminal administration is discussed. When butyrate occurs in the circulating blood a breakdown of liver glycogen is induced and the resulting hyperglycemia causes a decrease in the NEFA level. The butyric acid which is infused into or produced in the rumen is probably completely metabolized to ketone bodies in the rumen epithelium and depresses NEFA indirectly by increasing the production of insulin.