This study attempted to replicate previous demonstrations of classical conditioning of morphine analgesic tolerance, with the additional aim of determining whether stimulus overshadowing effects might explain previous conflicting findings. Eight groups of rats received a series of 10 morphine (5 mg/kg) and/or saline injections, differing only with respect to the contingency between a compound visual-auditory conditioned stimulus (CS) and the substance injected. When tested for analgesic responding to morphine in the presence of the compound CS, only those groups for which the CS and morphine injections were paired during the acquisition sequence evidenced tolerance. In a second experiment, tolerant animals were tested in the presence of one component of the compound CS. When a loud (85 dB) tone was used in the compound, less analgesic tolerance was elicited later by the weaker visual stimulus alone. This differential stimulus control of the analgesic response suggests that overshadowing may contribute to failures to replicate conditioned morphine tolerance. That internal morphine-produced stimuli might overshadow external cues is considered.