1. Previously it has been impossible to perform studies of human breast tissue in vivo. In this study, we investigated whether the microdialysis technique is applicable in human breast tissue and whether the concentrations of amino acids, lactate and pyruvate change during the menstrual cycle. 2. Microdialysis was performed twice during the menstrual cycle in eight healthy women, in the breast and subcutaneous fat. Amino acids, lactate and pyruvate were analysed by HPLC. 3. None of the women showed any complications, such as bleeding or infection, after the experiments. The concentrations of aspartic acid, asparagine, serine, glycine, threonine, tyrosine and ornithine were decreased in the breast late in the menstrual cycle. In the subcutaneous fat, the concentration of glycine decreased late in the cycle. 4. This study has shown that microdialysis is a safe technique and is suitable for investigations of human breast tissue. Our data indicate that metabolism in the breast changes during the menstrual cycle. The lower levels of specific amino acids late in the menstrual cycle could indicate a higher proliferative and/or apoptotic activity during this period. However, the data we have obtained cannot yet be fully explained.