A method for the chronical administration of morphine by the oral route is discussed. The method recommends the administration of morphine HCI dissolved in a 45% sucrose syrup and given orally for 4 weeks. The initial concentration of morphine in the syrup was 1 mg/ml and was increased weekly upto 4 mg/ml at the end of the experiment. This procedure rendered the animal physically dependent on morphine as observed by drugs withdrawal, when abstinence symptoms were easily identified. It became clear that morphine dependence phenomena (tolerance and the drug induced abstinence syndrome) in both rats and mice developed in association with, and were probably attributable to, change in the activity of choliergic neurons.