1. The passive electrical properties of muscle bundles obtained from the right ventricle of sheep or calf hearts were determined. Preparations were kept in silicon oil; through extracellular electrodes constant current pulses were made to flow between the ends of the bundles.2. Using micro-electrodes for potential recording, the following data were obtained: (i) a space constant of 880 mu; (ii) a membrane time constant of 4.4 msec; (iii) a ratio of intra-to-extracellular longitudinal resistance of 3.5: 1; (iv) a conduction velocity of 0.75 m/sec.3. The intracellular specific resistance (R(i)) in the longitudinal direction was 470Omega cm, corresponding to 3 times R(i) of Purkinje fibres or 9 times the specific resistance of Tyrode solution.4. A calculation of specific membrane resistance (R(m)) and capacity (C(m)) was up against uncertainties in estimating the surface area. Taking morphological data as obtained by light microscopy, R(m) works out at 9100Omega cm(2), C(m) 0.81 muF/cm(2). Electron micrographs suggest that the true surface membrane might be either larger (T-tubules) or smaller (tight junctions between parallel fibres) than the surface area as seen by the light microscope.5. The apparently small value of C(m) seems to indicate that the flow of current between ;outside' and ;inside' is restricted to only a fraction of the fibre surface, while a considerable part of the contact area between parallel fibres is of the low-resistance type. This would provide for functional connexions not only at the level of intercalated disks, but also along parallel-running fibres.