Abstract Subfebrile doses of bacterial pyrogens have been found to produce striking alterations in splanchnic circulation and metabolism. These consisted of a mean decrease in Bromsulphalein clearance of 33 per cent, a mean elevation of hepatic blood flow of 47 per cent, and a rise in mean splanchnic oxygen consumption of 18 per cent. Splanchnic glucose production was unaffected. The elevation of hepatic blood flow was not accompanied by proportionate increase in cardiac output. Contamination of parenterally injected materials with small amounts of bacterial pyrogens is, therefore, suggested as a possible cause for erratic alterations of splanchnic metabolism.