The effect of xylazine (Rompun) upon hepatic glucose production and blood flow rate was measured in two cows. Doses of 0-16 or 0-18 mg/kg bodyweight increased blood glucose concentrations by 200 per cent and hepatic glucose production by 400 per cent. Maximum blood glucose concentrations were reached approximately 40 minutes after dosing and did not start to fall until 185 minutes. Concentrations were near normal 24 hours after dosing. The increase in hepatic glucose production was greatest 20 minutes after dosing and production had returned to control rates 150 minutes after dosing. Visceral glucose utilisation was also increased. Blood flow rates in the hepatic and portal veins were reduced to 50 to 60 per cent of their predosing values. It is concluded that the prolonged hyperglycaemia which persists beyond 150 minutes is produced either by continued glucose production from sites other than liver and viscera or by reduced utilisation of the blood glucose by peripheral tissue.