The rabbit's nictitating membrane response was classically conditioned to tone and light conditioned stimuli presented for 800 ms before delivery of a 100-ms unconditioned shock stimulus. Both the mu receptor agonist morphine (5 mg/kg) and the kappa receptor agonist ethylketocyclazocine (1 mg/kg) significantly retarded the acquisition of conditioned responses (CRs). The retardant effects of both morphine and ethylketocyclazocine on CR acquisition could still be detected when the rabbits were tested 5 days after cessation of drug injections. At the dose employed in this study (5 mg/kg), the sigma receptor agonist N-allylnormetazocine had no effect on acquisition. The retardant effects of morphine and ethylketocyclazocine on acquisition were significantly antagonized by both naloxone (1 mg/kg) and N-allylnormetazocine (5 mg/kg). It was suggested that mu and possibly kappa receptors are involved in the retardant effects of opiates on the acquisition of classically conditioned responses.