Although there is comprehensive information on several parameters related to acute changes of the oxygen transport system in athletes, little information is available on chronic adaptations of the respiratory system at rest, as reflected by the out-of-competition venous blood-gas status. Such changes may represent markers of a more efficient oxygen metabolism at rest, associated with a superior physical recovery. Venous blood-gas status was investigated in two subpopulations of competitive athletes (47 male professional road cyclists, 72 male elite road cyclists) who had observed an 18-24-h resting period from the last training session, and 58 male sedentary blood donors. Significant differences were observed for the p50 values (the oxygen tension at which the haemoglobin is 50% saturated) between sedentary controls (25+/-1 mmHg) and either elite (26+/-1 mmHg; p<0.01) or professional athletes (27+/-0.8; p<0.01), whereas other conventional parameters of the venous blood-gas status (pH, P(O2), P(CO2) and saturation) did not differ significantly between the three study populations. Results of the present investigation suggest that intensive training may be associated with a right-shift of the standard oxygen dissociation curve, as attested by the increased p50. This rightward shift is usually considered a helpful adaptation, enabling a relatively more efficiently mechanism of oxygen delivery to the peripheral tissues, enabling physiological advantages during exercise and improved recovery.