Treatment of mice with either a 75-mg morphine pellet (72 h) s.c. or 100 mg/kg of morphine s.c. (3.5 h) did not alter the ED50 of (-)-N6-(phenylisopropyl)adenosine (PIA) in the tail-flick assay. Under the same treatment conditions, caffeine became a more potent antagonist of PIA-induced analgesia, and the dose-response curve for the locomotor effects of caffeine was shifted to the left. There was no change from control in the distribution of caffeine to the brain in mice pretreated with morphine. Brain levels of PIA were decreased significantly at the two highest doses used to elicit analgesia in morphine-implanted mice when compared to control. Adenosine receptor binding assays, utilizing [3H]PIA and [3H]diethylphenylxanthine as agonist and antagonist ligands, respectively, revealed significant increases in the Bmax values for both ligands without changes in Kd in morphine-implanted mice when compared to control. These data suggest that there is an increase in sensitivity to drugs which interact with adenosine receptors in morphine-tolerant and -dependent mice.