In this study we examined the effects of harmaline on Pavlovian conditioning of the rabbit's nictitating membrane response. The acquisition of conditioned responses was determined during a single session consisting of 120 pairings of a tone-conditioned stimulus with a corneal air puff unconditioned stimulus. Harmaline severely retarded (5 mg/kg) or completely blocked (10 and 20 mg/kg) acquisition of conditioned responses. The blocked or retarded acquisition of conditioned responses could still be detected when the rabbits were tested 2 d after cessation of drug injections, suggesting that harmaline was affecting acquisition and not the motoric expression of conditioned responses. Control experiments established that harmaline (5 mg/kg) did not affect (1) baseline levels of responding, (2) the level of non-associative responding to the conditioned stimulus, (3) the amplitude or any of the temporal characteristics of the unconditioned response, (4) the development of habituation to the unconditioned stimulus, and (5) the threshold of the unconditioned stimulus for eliciting the unconditioned response. However, harmaline did produce a 12 dB increase in the intensity threshold of the conditioned stimulus for eliciting conditioned responses. We concluded that the primary effect of harmaline was to impair stimulus processing within brainstem circuits such as to reduce the excitatory properties of the conditioned stimulus, thus retarding its entry into associative learning. The results were discussed with respect to the possible role of the inferior olive in associative learning.