Immunologic responses elicited after vaginal immunization of laboratory mammals are affected by the stage of the animal's estrous cycle at which antigen is delivered. The study reported here documents vaginal impedance as an improved means of determining accurately precise stages of the estrous cycle of guinea pigs. Impedance changes were correlated with cytologic changes observed in vaginal smears taken from the animals. The start of the impedance peak corresponded with proestrus, the rising side of the peak with estrus, and the apex of the peak with metestrus; diestrus was seen approximately 3 days after the peak apex. These results contrast with those of previous studies. Various dosage regimens of estradiol (1 to 1,000 micrograms/animal over 1 to 6 days) were used in guinea pigs in an attempt to invoke an extended (> 6 h) estrogen-induced mucosa. Parenteral administration of 1,000 micrograms of estradiol to guinea pigs daily for 6 days induced a 2- to 3-day estrus stage in these animals.