1. Response properties and passive electrical constants were assessed for caudate nucleus neurons in pentobarbital-anesthetized cats. The neurons studied were those which could be monosynaptically excited by substantia nigra and thalamic (centromedian-parafascicular) stimulation. 2. Input resistance and membrane time constant were estimated from the plateau value and time course, respectively, of the neuronal membrane response to intracellularly applied current pulses. The average values obtained were 16.5 Momega and 11.3 ms. Specific resistance and capacitance values were calculated. 3. Single or repetitive spikes were readily evoked by nigral or thalamic stimuli or by the application of direct depolarizing currents. Spike thresholds were higher for direct than for synaptic activation (7.2 vs. 5.6 mV). 4. Direct depolarizing stimuli with durations up to 600 ms elicited repetitive discharge with little adaptation of firing rate. The maximum discharge rates induced by direct stimuli were near 200 spikes per second. 5. The intracellular application of tetraethylammonium chloride (TEA) produced spike-prolongation effects in caudate neurons that were similar to the effects reported for other nerve membrane. 6. The probable identity of the recorded neurons as medium spiny neurons was discussed and, in addition, it was proposed that the characteristic silence of these cells is not likely due to intrinsic membrane specialization.