Differences between sexes and the effect of bilateral surgical gonadectomy on the response to morphine analgesia and dependence were examined in rats and mice. 2. The analgesic response to morphine (5 mg/kg) was assessed by the hot plate and the abdominal constriction tests. Dependence was induced by injecting morphine at increasing doses (5-160 mg/kg) for 6 consecutive days and withdrawal signs elicited by injecting naloxone (2.5 mg/kg). 3. The base line reaction times in the intact control, sham-operated and gonadectomized animals of either sex were not significantly different from each other. 4. After treatment with morphine, the percentage increase in the reaction time in gonadectomized male and female rats and in gonadectomized male mice was significantly higher than in their respective controls. 5. The increase in the reaction time, after morphine treatment, was significantly higher in gonadectomized female rats than in gonadectomized male rats. 6. Naloxone-induced withdrawal signs in morphine-dependent gonadectomized rats and mice were not significantly different from those in sham-operated controls. However, female rats in both groups exhibited a significantly higher number of escape jumping responses than males.
