Nandrolone (NDL) is an anabolic androgenic steroid known to be misused by elite athletes as well as some adolescents due to its muscle-building properties. We have shown that a 4-week period of repeated administration of NDL to 6-, but not 10-week-old rats enhances the developmental increase in grip strength without influencing developmental increases in body weight. Furthermore, systemic administration of morphine induced a smaller increase in dopamine (DA) efflux in the nucleus accumbens (NAc) of rats that had received NDL treatment for 4 weeks relative to those without NDL treatment. Recently, a single administration of NDL to mice was shown to enhance DAergic neural firing in the ventral tegmental area (VTA: Bontempi & Bonci, 2020). The DAergic neurons in the VTA are known to send projections to the NAc. Accordingly, we further analyzed the effects of a single administration of NDL on baseline as well as morphine-induced increases in accumbal DA efflux using in vivo microdialysis. Male Sprague-Dawley rats approximately 10 weeks old were used. A single administration of NDL (5.0 mg/kg s.c., 24 hr before morphine treatment) did not alter either basal levels of accumbal DA or the increase in accumbal DA efflux induced by systemic administration of morphine (1.0 mg/kg, s.c.). These results, together with our previous findings, show that prolonged but not a single administration of NDL in rats can inhibit morphine-induced increases in accumbal DA efflux.