The startle reflex is inhibited when the starting noise is preceded 30-500 msec by a weak prepulse. "Prepulse inhibition" (PPI) is reduced in specific neuropsychiatric disorders characterized clinically by impaired inhibition of sensory, motor, or cognitive information. PPI is sexually dimorphic, with men exhibiting significantly more PPI than women. We examined possible neuroendocrine substrates for this sex difference in PPI. The startle reflex, and a measure of visuospatial priming, were measured in 10 men, and in 46 normal women at different points in their menstrual cycle. In women, PPI was significantly reduced in the luteal vs follicular phase of the menstrual cycle. This reduction in PPI was most notable during the period corresponding to midluteal elevations of both estrogen and progesterone. In a task of visuospatial priming, follicular-phase women demonstrated a predominance of inhibition over facilitation, but this pattern reversed across the menstrual cycle.