Co-administration of ultra-low dose naltrexone (5 ng; NTX) with morphine (15 μg; MS) attenuated the loss in antinociception produced by morphine treatment alone in rats. The lower dose, 0.05 ng NTX did not attenuate the loss in antinociception produced by chronic morphine. Ultra-low dose naltrexone alone did not produce significant antinociception as compared to vehicle (saline) treated controls. Statistical analyses were performed using a two-way ANOVA followed by Bonferroni post hoc test. The asterisk denotes a significant difference from morphine-treated rats. ** = p < 0.01, *** = p < 0.001.