Sheep fed ad lib. on a good-quality pelleted diet (g/kg; hay 500, barley 300, molasses, fish meal and minerals) were infused via the hepatic portal vein with mixtures of the sodium salts of volatile fatty acids, acetate and propionate, and a variety of equivalent osmotic loads. Propionate infused at rates between 0·6 and 2·5 mmol/min consistently reduced food intake in a linear, dose-related manner. Propionate infusions resulted in consistent dose-related increases in peripheral venous plasma glucose concentration but variable changes in insulin concentration. Infusion of osmotically balanced mixtures of propionate, acetate, mannitol or saline (9 g NaCI/I) indicated that at constant osmotic loading propionate caused a greater reduction in intake than other infusions. Acetate infusions only depressed food intake when administered as a 1 M solution. Lower concentrations had little effect, similar to that of equivalent osmotic loads of mannitol or saline. It is concluded that portal propionate flow has a potential role in the control of food intake in ruminants which is independent of osmotic effects or changes in plasma insulin concentration.