A general problem found in the analysis of samples from brain microdialysis is the existence of some substances which can interfere with the dopamine. In our chromatographic conditions, we found an interfering peak with the following mobile phases: acetic acid / sodium acetate 1M (with 5% methanol and 1mM EDTA), and citric acid / sodium phosphate (with 5% methanol and 2mM EDTA). The solution for this problem was to employ a mobile phase consisting of KH2PO4(with lmM EDTA and 5% methanol), with an ion-pair agent of sodium salt of 1-octanesulfonic acid. The interfering peak was identified as the uric acid (resulting from the cellular metabolism) based on its chromatographic properties.