Two groups of naive male Sprague-Dawley rats, 5-7 and 24-26 months of age, were anesthetized with continuous intraperitoneal infusion of 4% chloral hydrate. Stimulation of the cerebellar vermis molecular layer permitted measurements of 12 different electrophysiological properties of parallel fiber Purkinje cell circuitry: parallel fiber conduction velocity, refractory period, threshold, and current dependent volley amplitude; slow negative wave threshold and current dependent amplitude; Purkinje cell activation threshold, latency, and current dependent spike driving; and Purkinje cell inhibitory threshold, latency, and current dependent duration of inhibition. Old subjects demonstrated deficits on all parameters except Purkinje cell inhibitory threshold. The relevance of these findings to our previous research on senescent changes in cell number, lipofuscin deposition, and spontaneous firing of Purkinje cells is discussed.