Adult male Wistar rats injected daily with 20 or 200 mg/kg morphine-SO4 for 35 days suffered a dose-dependent weight loss over the first 3 days of morphine withdrawal. However, during the next 28 days they gained weight more rapidly than controls, the rates being related to the previous morphine dosage. These findings were replicated in Sprague-Dawley rats treated for 26 days with 60 mg/kg morphine. Food-restricted controls suffering weight losses equal to those of the morphine-treated or morphine-withdrawn groups did not subsequently gain weight as rapidly as the latter groups. Therefore the rapid post-withdrawal weight gain may be a true adaptive response to the weight suppressing effects of morphine. Also, comparisons of weight changes during treatment in the two experiments indicated possible strain differences for tolerance to morphine's direct weight-reducing effect.