Estrogen exposure is a major risk factor for breast cancer. Tissue estrogen originates from the ovaries but a significant portion is also produced by enzyme activity locally in the breast itself. How these enzymes are regulated is not fully understood. The extracellular space, where the metabolic exchange and cell interactions take place, reflects the environment that surrounds the epithelium but there has been no previous study of hormone concentrations in this compartment. In the present study microdialysis was used to measure extracellular estrogen concentrations in breast tissue and abdominal subcutaneous fat in 12 healthy women in vivo. It was found that women with high plasma progesterone levels had significant increased levels of estradiol in breast tissue compared with fat tissue (breast tissue 168+/-6 pM; subcutaneous fat, 154+/-5 pM; P<0.05), whereas women with low plasma progesterone exhibited no difference. Moreover, there was a significant correlation between local breast tissue estradiol and plasma progesterone levels (r=0.709, P<0.01). There was no difference in estrone sulphate in breast and fat tissue regardless of progesterone levels. Estrone was not detectable. The results in this study suggest that progesterone may be one regulator in the local conversion of estrogen precursors into potent estradiol in normal breast tissue.