This study describes the simultaneous measurement of hepatic arterial and portal venous blood flow in the pentobarbital anesthetized rat by means of electromagnetic flowmeters. Hepatic arterial flow was 0.21 +/- 0.02 mL/min/g liver, and portal venous flow was 1.53 +/- 0.19 mL/min/g liver (n = 20). Flows remained stable for more than 3 hr. A clear advantage of the electromagnetic technique is that it allows the continuous simultaneous separate measurement of hepatic arterial and portal venous blood flow. Simultaneous measurement of hepatic blood flow by the electromagnetic method and the microsphere method yielded almost identical results. Adenosine infusion (100 micrograms/min) did not affect hepatic blood flow measured electromagnetically nor via the microsphere technique. The suitability of indocyanine green (ICG) as an indicator of hepatic blood flow was evaluated by comparing the ICG clearance at steady-state conditions to the values of liver flow obtained by direct electromagnetic measurement. ICG clearance was only 30% of the electromagnetically measured blood flow. These data strongly suggest that ICG clearance does not reflect liver flow in the rat. Intravenous infusion of the vasoactive agents phenylephrine (2, 4, 10 micrograms/min during 5 min) and adenosine (20, 40, 200 micrograms/min) did not affect portal venous nor hepatic arterial flow, measured by the electromagnetic method, although brisk effects on mean arterial blood pressure were observed. This suggests autoregulatory responses of the hepatic vascular bed. The data suggest that electromagnetic flowmeters may be used to measure portal venous and hepatic arterial flow simultaneously in anesthetized rats. The continuous measurement of both flows simultaneously offers a clear advantage over other methods of hepatic flow measurement.