- This study examined the effects of LSD-25 on the excitatory properties of auditory conditioned stimuli as a function of the interstimulus interval. The rabbit's eyeblink response was conditioned using a discriminative trace procedure by the pairing of a 500-msec auditory conditioned stimulus with a 100-msec shock unconditioned stimulus at intervals of 1000, 2000, 4000 and 8000 msec. Animals were able to acquire conditioned responses across all intervals. They then received doses of 35 or 85 micrograms/kg of LSD-25 prior to additional conditioning sessions. LSD-25 produced an increase in the magnitude of conditioned responses to both the positive and negative conditioned stimuli at all interstimulus intervals. It was concluded that LSD did not alter discriminative conditioning but rather enhanced the excitatory properties of both positive and negative conditioned stimuli.