Three experiments were conducted to compare the effects of LSD (30 nmol/kg) on the acquisition, maintenance, extinction and differentiation of the rabbit's classically conditioned nictitating membrane response. LSD significantly enhanced the acquisition of conditioned responses to tone and light conditioned stimuli as compared with vehicle injected controls (Experiments 1 and 2), but had no detectable effect on differential conditioning in Experiment 3. The conditioned responses acquired under LSD in Experiments 1 and 2 exhibited some unusual features in that: they were more rapidly extinguished under continued injections of LSD; they demonstrated a significant decrement when animals were switched from LSD to vehicle during maintenance; and they were virtually eliminated when animals were switched from LSD to vehicle during extinction. In contrast, conditioned responses acquired under saline injections in Experiments 1 and 2 were not affected when animals were switched to LSD injections during either maintenance or extinction. These results of Experiments 1 and 2 were interpreted as indicating that LSD produces an asymmetrical state-dependent learning.