The effects of sexual experience on morphine reward. Times spent in saline- (Sal) or morphine- paired (Mor; 0.5, 5 or 10 mg/kg bodyweight) chambers during the post-test in sexually naive (Naive, n = 10–13) or experienced (Exp, n = 9–13) males. Data presented as mean ± SEM; * indicates significant difference compared with Sal-paired chamber within same animals. NS, Not significant.