The main objective of the work was to evaluate the influence of a six day supplementation with sodium phosphate on circulatory and pulmonary variables, the level of 2,3-diphosphoglycerate (2,3-DPG) and the concentration of inorganic phosphates in blood serum of elite off-road cyclists. The research material included 19 cyclists which were randomly divided into a experimental group, supplemented with sodium phosphate and a control group receiving a placebo The subjects in the experimental group ingested sodium diphosphate in a dose of 50mg/kg of fat free mass per day. The supplement was ingested in even doses, four times per day. The control group received 4g of glucose in gelatin capsules (500mg), which were also divided into 4 even portions. During the experiment a significant (p<0.05) increase in maximal oxygen uptake was observed (VO2max), maximal minute ventilation (VEmax), as well as oxygen pulse (O2/HR). Also a significant decrease in resting and maximal exercise heart rate occurred. This was also true for each exercise load. A significant (p<0.05) increase in the serum concentration of non-organic phosphates (P) was observed which was accompanied by a decrease in serum calcium (Ca) concentration. The changes in the resting and post exercise concentration of 2,3-DPG were non significant, yet the supplementation procedure showed a tendency for increased level of this variable.