The opiate agonist 3H-etorphine was used to search for potential changes in in vivo opiate receptor binding in rats following chronic exposure to morphine sulfate. A rapid filtration method was employed to allow assessment of in vivo binding; receptor dissociation in vitro following in vivo labeling was also measured. No significant differences in total binding were seen with addicted animals, naive controls and naive animals pre-injected with morphine, at two different 3H-etorphine doses. In vitro dissociation under several conditions also yielded no differences. However, the rate of in vitro dissociation in the presence of both Na+ and a guanine nucleotide showed a small but significant decrease in dependent animals, suggesting a possible impairment of receptor effector coupling with morphine addiction.