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Mean heroin infusions for females (left; n=6) and males (right; n=6) at a constant dose of 0.0075 mg/kg/infusion. Females showed significantly lower heroin self-administration during the proestrus (P) phase of the estrous cycle. Males; however, did not show any differences in heroin infusions as a function of their female partner’s estrous cycle (**<.01).
