This study proposes to evaluate the changes in the serum levels of progesterone, etradiol, LH, FSH and prolactin during the estrous cycle of the Djungarian hamster during pregnancy and during a pregnancy block induced by mating with 2 males. The first experiment will elucidate the gonadotropic hormones during the estrous cycle with focus on the events surrounding ovulation, since this is the only short cycle rodent known that does not exhibit an increase in serum levels of progesterone on proestrus. The second experiment ascertains the gonadotropin and steroid changes in sera on proestrus associated with mating to a single male (normal pregnancy). Hormonal patterns will also be determined in early pregnancy. In addition the effects of mating to a single male on the number of ova shed and the timing of ovulation will be determined. The third experiment will ascertain the effects of mating females with 2 males on hormonal patterns. The double mating (with 2 males) causes a pregnancy block even though ovulation and behavioral estrus return 4 days later (no pseudopregnancy or prolonged cycle is observed). A histological assessment of the morphology of the corpora lutea and classification (staging) of follicles during the cycle, early pregnancy and pregnancy block will be assessed in order to characterize the luteal-follicular shift associated with the pregnancy block and subsequent return to estrus. These studies will provide insight into the effects of mating with one or two (or no) males on hormonal patterns and thus allow the development of a hypothesis as to the hormonal mechanisms associated with the pregnancy blockage.