Abstract. Electrolyte, cardiovascular, and metabolic changes due to the long-term mental stress of a chess tournament have been investigated by determining pH, pCO 2 , pO 2 , base excess, estimated free fatty acids (FFA), ionized Mg, and lactate from capillary blood and measuring pulse and RRsys in prospective losers or winners. It turned out that even before the commencement of the game, the later winners have metabolic Mg and cardiovascular data that differ significantly from that of losers [ 1 ]. By statistical analyses, Mg loss has been linearly correlated with increased FFA in losers, meaning that Mg loss also in this ambient is furthered by increased epinephrine levels. The linear correlations of pre- and post-game metabolism could be a basis for predicting results from extrapolating effort.