The aim of the study was to compare physiological responses between runners adapted and non-adapted to deep water running (DWR) at maximal and ventilatory threshold (VT) intensities. Seventeen (17) runners, either adapted to DWR (N=10) or non-adapted to DWR (N=7) participated in the study. Subjects in both groups did a maximal treadmill running (TDR) and deep water running (DWR) graded exercise test in which cardio respiratory variables were measured. Interaction between adaptation (adapted vs. non adapted) and condition (TDR vs. DWR) were analysed. The main effects of adaptation and condition were also analysed in isolation. Runners adapted to DWR had a lower reduction in VO 2 max in DWR in relation to their VO 2 max in TDR than runners non-adapted to DWR. VO 2 max, HRmax, VE max, VO 2 VT, HRVT, VEVT were significantly higher in TDR in comparison to DWR. No statistical differences were observed between TDR and DWR for RERmax, RPEVT, RERVT and %VO 2 VT/VO 2 max. Runners adapted to DWR presented a higher RER max and RER VT when compared to non-adapted runners. Therefore, it can be concluded that adaptation to DWR reduces the difference in VO 2 max between DWR and TDR, possibly as a result of the adapted runners recruit a larger muscle mass. However, the results of this study support previous findings with other populations demonstrating a lower maximal and submaximal physiological response on DWR for most of the measured parameters.