Our current studies indicate that the delayed puberty associated with reduced food intake, results from changes in the negative feedback system to estrogen rather than an alteration of the positive feedback response. The reasons for the altered negative feedback response will be further evaluated. Estrogen receptor activity will be measured in hypothalamic, pituitary and uterine tissue of prepubertal female rats kept on reduced caloric intake or protein deficient diets. Both total and translocatable receptor activity will be measured. The sensitivity of the pituitary tissue to GnRH will also be evaluated by in vitro methods both in the absence and presence of estrogen in the incubation medium. Finally, investigations will be initiated to evaluate the role of estrogen feedback in mediating the response of female rats to altered photoperiods. Negative and positive feedback sensitivity will be evaluated with different doses of estrogen in both short and long photoperiods.