Portal hypertension has been studied in the rat to see if it is associated to altered blood volume composition, as it has been shown in other species. Plasma volume was measured by isotope dilution using 99mTc labelled albumin in three groups of male Sprague-Dawley rats: normal rats (controls), partially ligated portal vein rats and rats with Cl4C induced cirrhosis. Plasma volume was significantly higher in rats with portal hypertension due to partially ligated portal vein and cirrhosis than in control animals. Similarly, the calculated blood volume was also significantly higher in the portal hypertensive animals than in control group. Portal hypertension in the rat, therefore, has been demonstrated to be associated to a marked hypervolemia and this finding should be taken into consideration in haemodynamic and pharmacokinetic studies in portal hypertensive rat models.