Abstract: Aims/Background: Diltiazem reduces systemic blood pressure by decreasing the vascular smooth muscle tone. In the liver however, diltiazem seems to cause vasoconstriction, as evidenced by increases in portal pressure. The questions raised by this observation are concerned with a) the site of action of diltiazem (large vessels or sinusoids), b) the formation of permeability barriers and c) the role of Ca2+. The experiments in the present study should provide an answer to these questions. Methods: The experimental System was the hemoglobin-free perfused rat liver. The multiple-indicator dilution technique was employed with simultaneous injection of [14C]sucrose and [3H]water. Mean transit times and distribution spaces were calculated from the normalized outflow profiles. Results: Calcium alone did not affect the hemodynamics of the liver. Diltiazem, however, changed several hemodynamic parameters when Ca2+ was present, but it was inactive in the absence of this cation. The hemodynamic effects of 500 μM diltiazem were: a) diminution of the transit time through the large vessels (to) and, consequently, of the accessible vascular space (66.9%); b) diminution of the mean transit time of [14C]sucrose (tsuc) and, consequently, of the accessible sinusoidal space (28.1%); c) diminution of the mean transit time of tritiated water (twater) and, consequently, of the accessible cellular space (68.9%); d) diminution of the cellular to extracellular space ratio (θ) from 1.42±0.05 to 0.46±0.11. Conclusions: The linear superposition of the tritiated water and labeled sucrose curves, predicted by Goresky's model, could be optimized even when the curves were obtained with diltiazem+Ca2+, indicating that the distribution of both tracer s was still flow-limited. The hemodynamic effects of diltiazem seem to be restricted to a vasoconstriction of the great vessels, an action which was strictly dependent on Ca2+. At the concentration of 500 μM, the effects of diltiazem were pronounced to the point of excluding completely about 2/3 of the liver parenchyma from the microcirculation, as indicated by the observed reduction in the accessible cell space. The sinusoids that were still supplied with perfusion fluid suffered considerable distension (2.19 fold) because the whole perfusate flow was deviated into the remaining 1/3 microcirculatory units. Diltiazem did not seem to induce the formation of intrahepatic shunts or diffusion barriers in the liver.