Acute and chronic morphine behavioral tolerance in hMOR-T and hMOR rats are the same. A. Acute tolerance was assessed by measuring changes in morphine analgesia induced by a high dose (30 μg/10 μl) of morphine given 18 hrs earlier. On day 2, the three rat groups exhibited a similar extent of reduction in morphine responses (saline: 41.4%, n = 4; hMOR: 40.4%, n = 3; hMOR-T: 43.3%, n = 3). B. Chronic morphine tolerance was studied in rats subjected to morphine treatment (hMOR-T: 1.5 μg/10 μl, n = 8; hMOR: 1.5 μg/10 μl, n = 4; saline: 5.0 μg/10 μl, n = 4) twice daily for 5 consecutive days. The chronic morphine-induced tolerance developed quickly and the rates of development among the rat groups were similar.