This study investigated whether manipulation of cardiovascular drift (CV drift) by changing exercise duration or by fluid ingestion is associated with altered peak oxygen uptake VO(2peak). METHODS: VO(2peak) was measured in 11 trained men immediately after they cycled at 60% control VO(2max) in 30 degrees C, 40% relative humidity for 15, 60, and 120 min with no fluid (15 NF, 60 NF, 120 NF) or 120 min with fluid (120 F). Stroke volume (SV), heart rate (HR), and related measures were measured in 120 NF and 120 F at 15, 60, and 120 min. RESULTS: Body mass decreased 0.7, 2.3, and 3.7% in 120 F, 60 NF, and 120 NF. SV at the end of submaximal exercise and VO(2peak) measured immediately thereafter were reduced significantly (P < 0.05) from 15-min values in 120 NF (13.8 and 8.7%) but not in 60 NF (4.6 and 1.2%) or 120 F (2.1 and 1.9%). CONCLUSIONS: The progressive decline in SV during prolonged, constant-rate submaximal exercise in a warm environment, reflective of increased cardiovascular strain associated with hyperthermia, dehydration, and other changes that occur over time, reduces VO(2peak). Fluid ingestion improves performance in prolonged exercise, in part, by mitigating the decline in SV and its determinants, and preserving VO(2peak).