Our objective was to validate the use of an ultrasonic transit-time flowmeter for the measurement of portal blood flow (PBF) in sheep. Data recorded with this technique were compared with those obtained with an indicator dilution method, and probes were calibrated in vivo. Wethers were fitted with catheters in the portal, jejunal, and ruminal veins and in a mesenteric artery. Ultrasonic flow probes were implanted around the portal vein: S-series probes in three wethers, and A-series probes in four wethers. The PBF measured with A-series probes was within 10% of that measured by indicator dilution, but PBF measured with S-series probes were 52 to 77% of that determined by indicator dilution. In vivo calibration indicated that A-series probes provided accurate measurement of PBF (absolute accuracy: 5% +/- zero flow error). In conclusion, an ultrasonic transit-time flowmeter, with an A-series probe, can be used to reliably measure PBF in sheep.