Mean transit times for the movement of extracellular and intracellular reference compounds through isolated perfused rat livers were determined during exposure of livers to platelet-activating factor (AGEPC; 1-0-hexa-decyl-2-acetyl-sn-glycero-3-phosphocholine) and the α-adrenergic agonist phenylephrine, using the multiple indicator dilution technique. From the outflow profiles of rapid bolus injections of 3H-sucrose and 14C-urea given to the liver, the estimated intracellular volume of distribution of small freely permeant substances, Vi, and Θ′, the ratio of intracellular to extracellular space, were computed. Exposure of the liver to AGEPC decreased Vi and Θ′ by 32 and 34%, respectively, from control values, whereas infusion of phenylephrine increased Vi by 16% and Θ′ by 33%. The results indicate that the hemodynamic effects of AGEPC in perfused rat liver cause the apparent loss of tissue space accessible to small permeant compounds. Phenylephrine, although increasing hepatic vascular resistance, measured at the portal vein, by the same magnitude as AGEPC, led to an increase in the apparent tissue space accessible to this same species.