Rats were injected with either morphine (5 mg/kg) or saline in association with one set of distinct environmental stimuli, and injected with saline in association with a different set of stimuli. After four conditioning trials, animals were given a 15-minute free-choice test to determine which stimulus environment was preferred. Animals displayed CPP as a significant increase in duration spent within the morphine-associated environment, but did not display any change in number of entries into that environment. In contrast, when extinction training was given following CPP, animals displayed a significant decrease in duration spent per entry into the morphine-associated environment, but did not display any change in total duration spent in that environment. These results suggest that assessment of the reinforcing conditioned response (CR) in the CPP model may require measurement of both duration spent in and number of entries into the drug-associated environment.