Blocking neuroprogesterone synthesis alters the pattern of estrous cycles in gonadally intact rats. The four day rat estrous cycle is diagrammatically presented. Each day of the cycle is indicated on the ordinate: D1 = diestrous day 1, D2 = diestrous day 2, P = proestrus and E=estrus. Neuroprogesterone synthesis was blocked by infusion of a P450scc inhibitor, aminoglutethimide (AGT), into the third ventricle (3V) via an implanted cannula. All animals treated with DMSO (5%, vehicle n = 14; data not shown) had normal 4 day estrous cycles as determined by vaginal cytology. In contrast, 11/14 AGT treated rats (0800 hrs on proestrus, indicated by the arrow) had disrupted estrous cycles with delayed onset of estrous as determined by vaginal cytology.