In earlier studies, it was shown that there were gender differences in several aspects of the pharmacological profile of morphine, including its antinociceptive activity, discriminative stimulus properties and its reinforcing effects. The purpose of the present studies was to examine whether there might also be gender-related differences in the development of physical dependence, as reflected in the expression of an opiate withdrawal syndrome upon cessation of chronic morphine administration. We found that a more severe spontaneous withdrawal syndrome was produced by chronic morphine injections or morphine pellet implantation in male rats than in females. The duration of the withdrawal syndrome was also longer. In contrast to our observations with spontaneous withdrawal, we found no gender differences in the naloxone-precipitated withdrawal syndrome induced by chronic morphine administration. These observations suggest that there are gender differences only in the expression of the spontaneous withdrawal syndrome, but not in the neuro-adaptive changes associated with the generation of physical dependence as reflected by naloxone-precipitated withdrawal.