We have recently shown that maintenance of portal hypertension in rats is highly dependent on high portal blood flow. This study attempts to determine whether portal hypertension, induced in the rat by partial constriction of the portal vein, can be reduced by lowering portal blood flow with a beta blocking agent. Portal hypertensive rats treated with propranolol had a reduction in portal blood flow to 0.89 +/- 0.13 ml X min-1 X gm-1 accompanied by reduction in portal pressure to 12.6 +/- 1.00 mm Hg, disproportionately small because of a rise in portal-collateral vascular resistance to 12.52 +/- 1.63 dyne X sec X cm-5 X gm X 10(5) accompanying the portal blood flow reduction. Similarly, portal hypertensive rats receiving beta-blockade with H35-25, an experimental beta-blocker, demonstrated a 49% reduction in portal blood flow accompanied by only a 14% reduction in portal pressure, also disproportionately low because of a complicating 77% rise in portal-collateral vascular resistance. Normal rats were then given saline infusion to achieve similar blood flow elevations to that of portal hypertensive animals.(ABSTRACT TRUNCATED AT 250 WORDS)