Hepatic artery resistance index (I.R.) was evaluated in six pigs through Doppler scan after progressive gauge reduction of the portal vein up to complete clamping. The I.R. values were (mean +/- S.D.): 0.48 +/- 0.01 in basal conditions; 0.37 +/- 0.03, and 0.33 +/- 0.01 with a reduction of 1/3 and 2/3 respectively; 0.27 +/- 0.02 after complete portal clamping. These data confirm the close relationship existing in the hepatic circulation between changes of the arterious resistence and those of the portal flow. The same animals, subsequently, underwent heterotopic liver transplantation in the right paracolic gutter. Immediately after surgery the I.R. values of the transplanted hepatic artery were recorded without clamping (basal portal flow 0.52 +/- 0.02) and after clamping of the transplanted portal vein (0.33 +/- 0.03), while maintaining clamped the recipient portal vein at the hilus. These values were comparable to those obtained in the recipient hepatic artery. These data suggest that, in spite of parenchymal damages secondary to surgical procedure and of the haemodynamically unfavorable site in which the organ is transplanted, heterotopic liver transplantation does not produce significant changes in hepatic arterious resistance.