Morphine tolerance was studied in pigeons (Columba livia, N = 9) trained to discriminate among a low dose of morphine (1.8 mg/kg), a high dose of morphine (10 mg/kg), and saline. Doses of morphine required for low-dose or high-dose stimulus effects were determined before, during, and after a 4-week treatment period, during which training was suspended. Treatment with 56 mg/kg, but not 10 mg/kg, morphine, b.i.d., increased the doses required for either low-dose or high-dose stimulus effects by approximately 10-fold. Both treatments increased doses required for rate suppression. Sensitivity recovered after a week of saline treatment. Acute treatment with 56 mg/kg morphine did not change sensitivity. These results suggest that chronic morphine treatment can produce surmountable, reversible tolerance to morphine acting as a discriminative stimulus, without disrupting a discrimination between low-dose and high-dose stimulus effects.