In this study, we examined changes in the event-related potential (ERP) to stimuli with and without reproductive significance occurring during the menstrual cycle. Eleven spontaneously cycling women were tested during three menstrual phases (menses, ovulatory phase, luteal phase) differing in plasma concentrations of gonadal hormones. ERPs were recorded while subjects were presented with slides showing pictures from four different stimulus categories (sexual stimuli, babies, people occupied with body care, ordinary people). Slides were presented randomly in the context of two tasks, requiring either affective processing (i.e., to judge the emotional content of a slide as positive, neutral, or negative) or structural processing (i.e., to estimate the number of parallel thin lines inserted in each picture). Menstrual phase primarily affected a late positive component (LPC) peaking 550–600 ms poststimulus. The effects were as follows: (i) During the ovulatory phase, amplitude of the LPC to sexual stimuli was larger than that evoked by the other stimulus categories. (ii) This relationship was not apparent during the other menstrual phases or (iii) during the ovulatory phase when the task required structural processing. The ovulatory increase in LPC positivity to sexual stimuli suggests a greater valence of these stimuli during a phase of increased sexual desire. The data indicate a specific effect of the menstrual cycle on the processing of sexual stimuli that increases with deeper emotional processing.