The dorsolateral pontine brain stem was investigated as a possible locus of neural elements mediating the classically conditioned nictitating membrane (NM) response in rabbit. Recording and brain stimulation were employed for this purpose. Low-impedance tungsten monopolar microelectrodes were chronically implanted into the pontine brain stem. Multiple-unit recording during classical conditioning revealed a conditioned increase in multiple-unit activity (MUA) which developed and extinguished concurrently with the acquisition and extinction of the behavioral conditioned response. Pseudoconditioning and conditioned inhibition controls indicated that the increase in MUA was an associative learning phenomenon. Histology indicated that electrode tips recording the CR-associated electrical activity were located mostly adjacent or dorsal to the motor trigeminal nucleus. Periocular shock pulses elicited short latency evoked responses throughout most of the dorsolateral pons, suggesting that information concerning the unconditioned stimulus is relayed to this region. Furthermore, electrical stimulation of this region produced a robust ipsilateral nictitating membrane response in a number of cases, suggesting that neural elements of the dorsolateral pons project to the motoneurons that produce membrane extension. A consideration of several criteria based on these experiments implicates a supratrigeminal zone [22] as containing the neural elements of dorsolateral pons most intimately associated with the conditioned NM response. Other interpretations, concerning fibers of passage through this region and