The purpose of this study was to investigate left-ventricular function during strenuous exercise with the self-contained breathing apparatus (SCBA). Using two-dimensional echocardiography, images of the left ventricle (LV) were acquired during sustained exercise (3 x 10-min) under two conditions: (1) SCBA, or (2) a low resistance breathing valve. Twenty healthy men volunteered for the study and in each condition subjects wore fire protective equipment. Heart rate, systolic blood pressure, cavity areas during systole and diastole (ESCA and EDCA, respectively), esophageal pressure, ventilation rate, oxygen consumption, perceived physical, thermal and respiratory distress and core temperature were measured at regular intervals. Urine specific gravity (< 1.020 g mL^-1) and haematological variables were used to infer hydration status. All subjects began both trials in a euhydrated state. No differences were found between conditions for heart rate, systolic blood pressure, ventilation rate, oxygen consumption, perceived distress, or any haematological variables. Peak expiratory esophageal pressure was always higher (P < 0.05) while EDCA and stroke area (SA) were significantly lower (P < 0.05) with the SCBA. ESCA, end-systolic transmural pressure (ESTMP), and LV contractility (ESTMP/ESCA) were similar between conditions. Sustained exercise with fire protective equipment resulted in significant reductions in EDCA, ESCA and SA from the start of exercise, which was associated with a 6.3 ± 0.8% reduction in plasma volume, increase in core temperature (37.0 ± 0.4 to 38.8 ± 0.3°C) and a significant increase in heart rate (146.9 ± 2.1 to 181.7 ± 2.4 bpm) throughout exercise. The results from this study support research by others showing that increased intrathoracic pressure reduces LV preload (EDCA); however, the novelty of the present study is that when venous return is compromised by sustained exercise and heat stress, SA cannot be maintained.