One of the most common uses of capillary electrophoresis with conductometric detection is the analysis of different samples containing alkaline and alkaline earth metals. However, the high sodium content, which is a very common occurrence, may cause loss of resolution of some peaks. In this work, the conditions and a running buffer suitable for serum and hemodialysis fluid analyses are presented. The approach basically consists in the reduction of mobilities of Ca2+ and Mg2+ by addition of methanol and lactate in the running buffer. Limits of detection in the range from 0.5 to 1.0 mumol L-1 with molar Na+/Ca2+ ratios as high as 1000 were obtained, which are conditions that exceed the requirements of the proposed determination.