Membrane electrical constants have been studied in neurons of the red nucleus (RN) of the cat which were identified antidromically from the spinal cord. For each cell, the input resistance was determined from the membrane potential changes to current steps and was found to be 2.5+/-0.9 Momega in twenty five RN cells studied. In addition, linear summation of the membrane responses induced by two current pulses was demonstrated. 2. From the membrane voltage transients to current steps, the first membrane time constant, tau0, and second time constant, tau1, were determined as 5.6+/-1.0 msec and 0.6+/-0.2 msec, respectively. The ratio of the amplitudes of two exponential functions, E1/E0, was 0.18+/-0.05. A linear relation was found between the ratio of these amplitudes of exponential functions and that of the two time constants. 3. The cable parameter (electrotonic length, L) of the combined soma and dendrites of the RN neutrons was estimated as 1.1 from membrane transient data using the relation developed by Rall (1969). 4. By using this parameter, an attempt was made to estimate the location of the two excitatory inputs on the soma-dendritic membrane of RN cells.