To investigate the role of catecholamines (CA) in increased portal pressure following the portal vein ligation, we measured free plasma CA (norepinephrine:NE, epinephrine:E) concentrations serially for five hours (in 30 minutes interval) in 26 portal vein ligated rats and 5 sham-operated rats, using microdialysis system. We employed the microdialysis technique in conjunction with high-performance liquid chromatography, since being measurable for only small amount sample (10 μl) to obtain plasma CA levels. A coaxial microdialysis probe covered by a dialyzing membrane 3.0 mm in length and 0.5 mm in diameter was inserted into the portal vein at the post ligated site. A ligature of 3.0 silk was tied around the portal vein with a metal needle 1.2 mm i.d. and then the needle was removed. Binding-free NE increased almost twice to 15.9±1.8 (mean ± SEM) pg/60 μl /30 min 0.5 hour after (P<0.05), and thereafter increased more triplicate to 30.1±5.5 (P<0.01) 5 hours after ligation from 9.6 ± 2.1 of the preligation baseline value. Binding-free E increased, in same manner to NE, triplicate to 23.1 ± 11.3 significantly 5 hours after ligation from 7.4 ± 1.5 of the preligation value. We did not observed any significant increases in these plasma levels from those of ligated animals in sham-operated rats. The results suggested that postligated increase in portal pressure and/or progress in colateral circulation could be related to the endogenously enhanced catecholamines and increased hepatoportal sympathetic nervous activity.