In order to ascertain the effect of modified supplementation of electrolytes and trace metals on intracellular uptake, 75 patients undergoing mitral valve replacement were randomly divided into three groups: group A (n = 25) received potassium and sodium chloride while group B (n = 25) was treated with potassium aspartate; in group C (n = 25), a balanced solution of electrolytes and trace metals (Inzolen) with aspartate as the anion was given. The treatment in the three groups was administered on the basis of frequently measured plasma levels of potassium and sodium. Anesthesia was similar in all patients; cardiac arrest was performed by crossclamping of the aorta using "Bretschneider's cardioplegia" for sodium withdrawal. After removal of the mitral valve, a sample of papillary muscle was obtained and analyzed by atomic absorption for its content of sodium, potassium, magnesium, zinc, and copper (Na, K, Mg, Zn, Cu). Sodium levels in papillary tissue were below the normal range in all groups and without differences depending upon the kind of treatment. Potassium levels in group A, however, (KCl) were markedly below those in groups B and C (aspartates). A similar effect could be observed with respect to tissue levels of Mg, Zn, and Cu. Supplementation of trace metals (group C) had no additional effect on tissue concentrations: there were no significant differences between group B (K aspartate) and group C (K-, Mg-, Zn-, Cu-, Na aspartate). Our results stress the importance of effective treatment of electrolyte and trace metal deficiencies. The present data suggest that the utilization of intracellular cations can particularly be improved by using solutions with aspartate as the anion.