Abstract In this study, rats were trained to discriminate fentanyl from saline at training doses of 0.0025, 0.005, 0.01, 0.02 and 0.04 mg/kg. It appeared that the training dose affected the length and the shape of the acquisition curve, and the asymptotic level of discriminative response control. The ED 50-value of fentanyl and of morphine to induce stimulus generalization with the fentanyl training dose was proportional to this dose. However, the generalization gradient for both fentanyl and morphine became shallower as. the training dose was lower; this effect may reflect the deterioration of response control at lower training doses. Furthermore, the training dose co-determined the degree to which cyclazocine and d-amphetamine induced stimulus generalization with a narcotic agonist training drug, and also the degree to which naloxone antagonized the cuing properties of the training drug. Fentanyl training doses equal to or higher than 0.02 mg/kg seem to be required to produce a discriminative stimulus complex which is highly specific for narcotic drugs; the discriminative stimulus properties of lower training doses of fentanyl possess increasingly less pharmacological specificity. It is concluded that the training dose of a narcotic agonist contributes critically in determining the acquisition of discriminative response control as well as various quantitative and qualitative aspects of stimulus generalization with the training drug condition.