In this study a technique for insertion of microdialysis catheters and the influence of the position of the catheters within normal human breast tissue were evaluated by measuring amino acids. Moreover, to assess variability over time, the levels of amino acids were measured during a period of 3 h. In nine healthy women two parallel microdialysis catheters were implanted, guided by a catheter for intravenous use, into the breast tissue. All insertions were successful and there were no complications. The levels of amino acids were equal in the two parallel catheters and varied less than 10% over a period of 3 h. Insertion of the microdialysis catheter via an intravenous catheter is suitable for the dense breast tissue. The position of the microdialysis catheter within the same breast seems to be of minor importance for measurements of amino acids. Thus, the described technique is a safe and reproducible way of investigating the human breast in vivo.