Through the use of a chronic microdrive recording system, neuronal unit activity was recorded throughout the brainstem of the rabbit during performance of the classically conditioned nictitating membrane (NM) extension/eyeblink response using an acoustical conditioned stimulus (CS) and a corneal airpuff unconditioned stimulus (UCS). Regions which exhibited neuronal responses near the onset of the learned response were found to be relatively localized to: the abducens, accessory abducens, and facial nuclei (the motoneurons known to innervate the muscles responsible for expression of the conditioned response); the sensory nuclei of the fifth (probably representing somatosensory or proprioceptive feedback from the conditioned response); the superior colliculus; the periaqueductal gray; various reticular regions and the brainstem nuclei directly connected with the cerebellum (pontine nuclei, tegmental reticular nucleus (Bechterew), red nucleus, and perhaps the inferior olive). Stimulus (tone-airpuff) evoked responses were found within all classical auditory nuclei of the brainstem; the superior colliculus; the periaqueductal gray; pontine nuclei; fifth sensory nuclei, and various reticular regions. Recent lesion studies have shown the ipsilateral cerebellum to be essential for the learning and retention of this response. Collectively these results indicate that the cerebellum and its related brainstem nuclei are critically involved in the control and production of the classically conditioned NM/eyeblink response and may contain essential long term neuronal changes--the 'memory trace'--which serves to encode this learned response.