This investigation assessed the effects of immediate or delayed re-hydration post-exercise, on mean arterial blood pressure (MAP) and on blood plasma volume (PV) expansion post-exercise. It was hypothesised that fluid ingestion would raise MAP and attenuate PV expansion. On two occasions separated by seven days, eight males (age 20.4 ± 1.7 years, mass 79 ± 5 kg [means ± SD]; VO_2max 48 ± 11 mL·kg^-1·minute^-1, [mean ± SE]) cycled in the heat (35°C, 50% relative humidity) at a power output associated with 50% VO_2max, until 1.0kg body mass was lost. 1L water was given either immediately thereafter, or two hours post-exercise by random assignment. On both occasions, MAP was calculated every five minutes for a period of 30-minutes post-exercise, and change in PV was calculated 24-hours post-exercise. Repeated measures ANOVA for MAP results suggested a low probability of a treatment effect (p = 0.655), a high probability of a time effect (p = 0.006), and a moderately high probability of a time x treatment interaction (p = 0.076); MAP tended to be lower when fluid had been consumed. PV expansions 24-hours post-exercise were not significant changes with respect to zero, and were not significantly different by treatment condition. In conclusion: (a) The exercise was not sufficient to elicit significant PV expansions; thus, we were unable to determine the effects of the timing of post-exercise re-hydration on PV expansion. (b) The hypothesis regarding MAP in response to drinking was not supported, rather there was a 92% probability that the inverse affect occurs.

• Post exercise hypotension is perhaps the most important mediator of plasma volume expansion post exercise

• It was hypothesised that drinking water immediately post exercise would attenuate post exercise hypotension by rapidly ameliorating dehydration

• We found that not only was our hypothesis incorrect, but rather a 92% probability exists that the inverse is true, i.e. drinking water in fact leads to lowered blood pressure, as compared to not drinking.