The purpose of this study was to investigate the discriminative stimulus properties of morphine and codeine using different administration routes to that used at drug discrimination training. Rats were trained to discriminate morphine at 3 mg/kg from saline by the intraperitoneal route in a standard two-lever drug discrimination paradigm. Generalization of morphine by the subcutaneous and the oral routes, and codeine by the intraperitoneal and the oral routes to the discriminative stimulus properties of the morphine training dose were investigated. Morphine at 3 mg/kg by the subcutaneous route generalized to the morphine training dose and 10 of 12 rats showed 80% or more morphine-lever responses. In the administration of morphine by the oral route, morphine at 30 mg/kg generalized to the morphine training dose and all rats showed 80% or more morphine-lever responses within the range of 3 to 30 mg/kg. In the administration of codeine by the intraperitoneal route, codeine at 20 mg/kg generalized to the morphine training dose and 14 of the 15 animals showed 80% or more morphine-lever responses within the range of 3 to 20 mg/kg. In the administration of codeine by the oral route, codeine at 60 mg/kg generalized to the morphine training dose and 14 of the 15 animals showed 80% or more morphine-lever responses within the range of 10 to 60 mg/kg. Thus, the discriminative stimulus properties of morphine and codeine were comparable when using different administration routes to those at discrimination training.