Because opioid antagonists acutely produce rises in serum beta endorphin, we studied beta endorphin levels in 21 former opiate addicts chronically taking naltrexone. The mean AM (19.5 pg/ml) beta endorphin level was higher than the AM mean for 39 normals under 40 years old (12.1 pg/ml) (t = 3.2, p less than 0.001); the mean PM level for the naltrexone treated patients was 13.6 pg/ml. Four patients had beta endorphin levels more than 2 S.D. above the mean for the normals (greater than 26.4 pg/ml), and six others had relatively elevated PM levels. Thus, 47% (10/21) had abnormal patterns of beta endorphin levels. We had previously reported abnormally high cortisol levels in these patients, and AM cortisol correlated with AM beta endorphin levels (r = 0.7, p less than 0.001). We concluded that sustained beta endorphin elevations may occur during chronic naltrexone treatment.