Animals previously trained to criterion on an eight-arm radial maze task received either bilateral 6-hydroxydopamine lesions of the dorsal noradrenergic bundle (DNB) or control surgery. Following a 3-week recovery period, the animals were trained on the same radial maze in two novel environments. By a within-subjects design, in one of these environments animals received posttraining systemic treatment with the opiate antagonist naloxone; in the other novel environment, they received vehicle injection. In animals that received control surgery, opiate antagonist treatment produced a reliable enhancement of performance. Although the DNB-lesion animals did not differ from the control-surgery animals under the saline treatment condition, denervation of forebrain norepinephrine (NE) was found to prevent the memory enhancing effect of posttraining naloxone administration. These results provide further support that enhanced retention obtained with opiate antagonist administration is dependent upon intact NE function.