Previous studies have shown that progesterone concentrations in breast fluid are correlated with those in serum. According to preliminary data, this is not true of estradiol, which is lowest in nipple aspirate fluid (NAF) at midcycle when serum levels are maximal. The present study is designed as a longitudinal investigation into the patterns of estradiol in serum and NAF. Factors that may influence the upake will be studied; among these are progesterone and vascular endothelial growth factor (VEGF). Women volunteers will be recuited from the Love/Avon Army of Women. The subjects will learn to obtain NAF in the clinic and will collect NAF on three occasions during the menstrual cycle (at 7 day intervals) at home. They will also collect urine specimens daily for 23 days. Menstrual bleeding dates will be recorded. NAF will be analyzed for progesterone, estradiol, estrone, and VEGF. Urine will be analyzed for creatinine, pregnanediol, and estrone sulfate. The changes in these analytes will be determined statistically in ovulating women between the weekly intervals.