Exercise and exogenous heat each stimulate multiple adaptations, but the role of the related stressor, dehydration, is unclear. While severe hypohydration potentially “silences” the long-term heat acclimated phenotype (in rats) [1], mild-to-moderate dehydration may stimulate cardiovascular adaptations [2, 3]. We tested the hypothesis that exogenous heat stress and dehydration can additively potentiate short-term haematological adaptations to exercise. Five male and five female volunteers (mean ± SD: 173 ± 11 cm; 72.1 ± 11.5 kg; 24 ± 3 y) completed three trials in randomised order, involving 90-min orthostatically-stressful callisthenics, in: i) temperate (~22˚C, 50% rh; Con); ii) exogenous heat (~40˚C, 60% rh) whilst euhydrated (Hot+Euhy), and iii) exogenous heat with dehydration (12 h prior fluid restriction then permissive dehydration; Hot+Dehy). Time of day, menstrual phase, and activity levels were standardised. Measures recorded during exercise and intermittently until 24 h thereafter were rectal temperature, heart rate, mean arterial pressure (MAP) and Δ plasma volume. Plasma erythropoietin conc. (EPO) was measured before exercise and at 0 and 2 h after exercise. Compared to Con, core temperature tended to rise more in Hot+Euhy (by 0.48˚C; 95% CI: -0.06 – 1.01˚C: Cohen’s d=2.1; p=0.07), and more so with dehydration (by 0.47˚C for Hot+Dehy vs. Heat+Euhy; 0.18 – 0.76˚C; d=3.8 p=0.007; from ANOVA). Heart rate did likewise: (mean ± SD) +32 ± 7, +80 ± 30 and +95 ± 27 b/min for Con, Hot+Euhy, and Hot+Dehy respectively (all p<0.05). Post-exercise hypotension at 24 h was larger with the addition of heat (by 3 mm Hg for Hot+Euhy vs. Con; 1 – 5 mm Hg; d=0.9; p=0.03), and more so with dehydration (by 2 mm Hg for Hot+Dehy vs. Hot+Euhy; 1 – 3 mm Hg; d=2.1; p=0.008). Similarly, the hypervolaemic response at 24 h was larger with heat (by 4% for Hot+Euhy vs. Con; 1.9 – 6.9%; d=0.7; p=0.01), and more so with dehydration (by 8.8% for Hot+Dehy vs. Heat+Euhy; 3.2 – 14.3%; d=0.9; p=0.006). EPO was unchanged from baseline in any trial: 0.5 ± 2.2; -2.0 – 1; and -1.0 ± 3.2; -3.2 – 1.3 mU/mL change in Con. from baseline to post, and 2 h respectively; -0.9 ± 2.1: -2.8 – 0.9; and -1.6 ± 1.2: -2.7 – 0.6 mU/mL change in Heat+Euhy from baseline to post, and 2 h respectively; and 0.7 ± 2.0: -0.6 – 2.1; and 0.2 ± 4.1: -2.6 – 3.1 mU/mL change in Heat+Dehy from baseline to post, and 2 h respectively. Trials were pooled for correlational analysis. The rise in core temperature was strongly and negatively related to the hypotension at 24 h (r = -0.7; p<0.0001), which was similarly related to the hypervolaemia at 24 h (r = -0.7) The rise core temperature was also strongly and positively related to this hypervolaemia (r = 0.7). In conclusion, transient dehydration appeared to potentiate haematological (hypervolaemic) and cardiovascular (hypotensive) responses following exercise.