The McCollough effect across the menstrual cycle. The McCollough effect (ME) has been shown to be sensitive to cholinergic agents, being strengthened by hyoscine (antagonist) and weakened by physostigmine (agonist), and possibly to more generalized changes in CNS arousal. We therefore expected the ME to be sensitive to hormonal changes during the menstrual cycle, being strongest in the postovulatory phases when arousal is low. In two experiments we found a highly significant effect of menstrual phase for the normally cycling women, but not for oral contraceptive users: ME strength gradually increased across the cycle, reaching a premenstrual peak. These findings may be explained in terms of hormonally mediated changes in arousal across the menstrual cycle.