To investigate the effects of descent from moderate altitude combined with exposure to high ambient temperatures on plasma volume, a repeated measures design was used in young male cyclists (n=10). The cyclists (age: 18.2±1.2 y and maximal oxygen uptake​:​​​​​​ 62.0±4.1 mL/kg/min), who lived at an altitude of 2,600 m, trained at a lower altitude (450 m) for 7 days. Hematological variables were measured prior to descent, at low altitude, and 1 day after returning to 2,600 m. Hemoglobin mass was measured before descent and used for the indirect calculation of plasma volume. Hemoglobin concentration decreased from 17.1±0.7 g/dL to 15.9±0.6 by day 1 and to 15.4±0.3 g/dL by day 5 (both p<0.001) and then increased to 16.5±0.7 g/dL the day after returning to the initial altitude. Plasma volume increased from 2,946±463 mL by 12.9±6.6% on day 1, reached a maximum of 16.2±9.7% on day 5 (both p<0.01), and decreased again to+7.4±9.5% after returning to the altitude. The change from moderate to low altitude leads to a rapid increase in plasma volume, which can be expected to affect performance by reducing oxygen transport capacity and increasing maximal cardiac output. Additionally, the athlete’s biological passport may be affected erroneously.