The macroscopic resistivity of canine ventricular tissue has been measuredin vivo. Using a tetrapolar intramural electrode, values have been obtained for transmural resistivity as a function of depth, phase of the cardiac cycle and of the frequency of the applied current. The time course of resistivity owing to induced local ischaemia has been recorded. Using epicardial electrodes, the amount of anisotropy of the macroscopic resistivity has been estimated. The results obtained put the transmural resistivity at about 410 Ωcm and show it to be independent of the cardiac phase, independent of frequency and independent of depth. Induced local ischaemia was found to be followed by a substantial increase in the resistivity with time. The amount of anisotropy observed epicardially shows the ratio of transverse to longitudinal resistivity to be of the order of 2. These results are discussed in relation to a cylindrical model of the underlying fibre structure.