The presentation of a neutral or conditioned stimulus (CS) at an appropriate interval prior to the presentation of a corneal airpuff, or a paraorbital shock (unconditioned stimulus, US) can facilitate the amplitude of the unconditioned nictitating membrane (NM) response in rabbit. In two experiments, it was demonstrated that an associative process mediates the maintenance of that facilitation during repeated CS-US pairings. Although CS-alone presentations produced a substantial decrease in the amount of reflex facilitation in animals not pretrained with the CS, pretraining that consisted of paired CS-US presentations prevented that decrease when CS-alone presentations were subsequently given. Conditioned facilitation of the unconditioned response occurred very rapidly (within 5-12 trials in these experiments) and long before the appearance of overt conditioned responses to the CS. In addition, it was demonstrated that conditioned facilitation can be relatively specific to the tonal frequency of the CS. These results indicate the first sign of conditioning of the NM response is exhibited in the amplitude of the unconditioned response.