The aim of the study was to evaluate the relationship between exercise tolerance, glucose and insulin levels and biochemical parameters related to endothelial function [endothelin-1 (ET-1), nitrite/nitrate (NOx)] in patients with cardiological syndrome X in comparison with healthy volunteers. The decrease of NOx level and NOx / ET-1 ratio suggests that endothelial dysfunction is present in cardiological syndrome X. Exercise tolerance correlated positively with NOx and negatively with insulin concentrations.