A partially characterized mouse brain endorphin was shown to be elevated (p 0.01 U-Test) in ICR strain mice that had been made tolerant but not dependent upon morphine (5 mg/kg sc., given for 32 days). The animals were tolerant to the antinociceptive effect of morphine as judged by the tail immersion assay (48 degrees C) but showed no detectable dependence or withdrawal syndrome effects following the administration of naloxone (writhing, jumping, diarrhea or hypermotility) on day 33. No significant changes were seen in any other mouse brain endorphins (p 0.05 U-Test). Also there was no apparent change in the number of binding properties of 3H-dihydromorphine (3H-DHM) receptors (mu-receptors) in chronic morphine (CM) treated, as compared with chronic saline (CS) treated animals.