1. Unit activity and excitability were studied at the midlateral and suprasylvian cortex of naive, blink-conditioned and "randomization" cats. The latter received the same CS and US as did the conditioned animals, but in random temporal order and with random intertrial intervals with mean comparable to that used for conditioning. The randomization group failed to develop a blink CR. 2. With conditioning, spontaneous and evoked unit discharges were increased above levels found in naive animals. Correspondingly, levels of extracellularly injected current required to elicit a spike discharge were lower in conditioned than in naive animals. In addition, in the conditioned animals, the degree of enhancement of evoked activity and excitability was found to be greatest in the units that responded to the CS, as opposed to units that responded to another auditory stimulus of equal intensity but of no special behavioral significance vis-a-vis the conditioned reflex. 3...