Studies in animal models suggest that sugar deprivation following excessive intake elicits some opioid-like withdrawal signs. In the present study, opioid-like effects of excessive sucrose intake were further characterized in C57BL/6 mice by comparing the effects of the opioid antagonist naloxone on food-reinforced responding before and during sucrose availability and, in parallel experiments, following chronic morphine administration. Results show that naloxone produced time-dependent and dose-dependent decreases in operant response rates after 4 weeks of excessive sucrose consumption, and that these effects were comparable with those produced by chronic morphine injections. These findings extend the observation that excessive sucrose consumption may produce opioid-like withdrawal signs, and suggest that operant assays of withdrawal-suppressed behaviors may be useful for further study of excessive sucrose consumption.