The estrous cycle, which has been studied extensively in rodents, refers to the cyclic alterations that occur in the female reproductive tract and in sexual receptivity. The estrous cycle is often likened to the menstrual cycle in humans. It is composed of four distinct stages – proestrus, estrus, metestrus, and diestrus – that are each characterized by distinct physiology as well as animal morphology and behavior. Proestrus is the portion of the cycle when eggs reach full maturity within the follicles. External examination of the female genitalia reveals a swollen vulva and open vagina. Estrus is the stage of the cycle when ovulation occurs. The vagina remains open, and, at this stage, females are maximally receptive to males. Metestrus is the stage when the mature egg moves through the fallopian tube, and the vagina is closed. If successful copulation and fertilization occur, pregnancy will occur. If pregnancy does not occur, diestrus will ensue. Diestrus is the final stage of the estrous cycle in which unfertilized eggs are eliminated, and the vagina remains closed. During this stage, a new cohort of follicles begins rapid growth to prepare for ovulation in the next cycle.