A modified conductance method to determine the cross-sectional areas (CSAs) of arteries in piglets was evaluated in vivo. The method utilized a conductance catheter having four electrodes. Between the outer electrodes an alternating current was applied and between the inner electrodes the induced voltage difference was measured and converted into a conductance. CSA was determined from measured conductance minus parallel conductance, which is the conductance of the tissues surrounding the vessel times the length between the measuring electrodes of the conductance catheter divided by the conductivity of blood. The parallel conductance was determined by injecting hypertonic saline to change blood conductivity. The conductivity of blood was calculated from temperature and hematocrit and corrected for maximal deformation and changes in orientation of the erythrocytes under shear stress conditions. The equations to calculate the conductivity of blood were obtained from in vitro experiments. In vivo average aortic CSAs. determined with the conductance method CSA(G) in five piglets, were compared to those determined with the intravascular ultrasound method CSA(IVUS). The regression equation between both values was CSA(G)=-0.09+1.00 x CSA(IVUS), r=0.97, n=53. The mean difference between the values was -0.29%+/-5.57% (2 standard deviations). We conclude that the modified conductance method is a reliable technique to estimate the average cross-sectional areas of the aorta in piglets.