Rabbits, chronically implanted with recording electrodes in the cerebellar interpositus nucleus and following acquisition of a classically conditioned eyelid response, were injected with haloperidol (HAL, 250 micrograms/kg). HAL significantly reduced the number of conditioned responses when a 75 and 85 dB tone conditioned stimulus (CS) was presented but not when a 95 dB tone CS was used. There was a corresponding decrease in interpositus activity at the 75 and 85 dB CS intensities but not at the 95 dB intensity. HAL appeared to disrupt CRs and interpositus activity by increasing the intensity threshold of the tone CS for eliciting conditioned responses. Possible mechanisms for the effect of HAL on neural circuitry involved in classical eyelid conditioning are discussed.