These experiments were conducted to examine extinction and retention of a classically conditioned response in acute spinal cats. Conditioning was obtained by pairing a mild electrical stimulus to the superficial peroneal sensory nerve (CS) with a stronger electrical stimulus to the ankle skin (US) of the same leg. Subsequent extinction was produced by presenting CS-alone trials. In the first experiment, animals given massed extinction trials showed response decrements to base levels. Response decrements were not found in animals that received distributed extinction trials. In the second experiment, .5-, 1-, 2-, 3-, or 4-hr intervals between acquisition and extinction produced no significant differences in the extinction data. In the third experiment, animals received extinction trials immediately or 30 min after acquisition trials, followed by 20 additional extinction trials 30 min later. Analyses of these data indicated significant acquisition and extinction in the 10- and 20- acquisition trial groups. As in Experiment 2, varying the interval between acquisition and extinction did not produce any group differences in the extinction data. The results of these experiments demonstrate that response increases produced by paired trials in the spinal preparation do not decay spontaneously over time and are not caused by sensitization effects.