Guidelines for rehydration following exercise typically advise drinking 120-150% of the body mass lost (ΔBM), depending on sodium ingestion. No allowance is made for the possibility that substantive mass loss in exercise may arise from glycogenolysis (substrate loss plus water release; [1]), which would presumably require rehydration at a rate more commensurate with glycogenesis [2]. Renal reabsorption of sodium and water might also benefit from less aggressive rehydration [3, 4]. We tested the hypothesis that BM recovers more slowly following hypohydration incurred by exercise than by heat, whereas plasma volume would expand more rapidly following exercise. Eight physically-active volunteers (6 males, 2 females; BM 72.5 kg (9.3); mean (SD)) dehydrated to at least 3% BM passively in the heat (40.3 °C) or via high-intensity-interval cycling (29.5 °C), in randomised order, separated by 28 d for females or >5 d males. Subjects then sat in comfortable conditions (26.4 °C) to ingest 120% of their ΔBM as rapidly as comfortably possible; ¾ as a sports drink (5.6% carbohydrate, 24 mMol/L sodium) and ¼ as water. Fluid balance was recorded for 2 h, after which they left the lab and recorded ad libitum-drinking and micturition volumes until returning the next morning. The ΔBM averaged -3.6% (0.5) in EX-HYPO and -3.7% (0.8) in HEAT-HYPO (t-test: p=.59), while plasma osmolality was 293 (3) and 293 mOsmlol/kg (5) (p=.83). Drinking duration was 66 min (38) in EX-HYPO and 85 min (37) in HEAT-HYPO (p=.09). Urine production during this initial 2 h was larger in EX-HYPO (496 mL (124)) than in HEAT-HYPO (336 mL (202); p=.05; mean ±95%CI difference: 160 ±160 mL), and was also less variable (range: 285 to 605 mL, vs 40 to 690 mL). Urine volume thereafter was similar between EX-HYPO (2093 mL (303)) and HEAT-HYPO (2118 mL (889); p=.93), as was the gain in BM from baseline (0.65 (0.40) vs 0.47 kg (0.62); p=.51). Plasma volume exceeded its resting baseline by 5.6% (5.9) at 1 h, and by 8.6% (2.0) at 2 h in EX-HYPO, but not in HEAT-HYPO (-1.7% (7.8) at 1 h and 0.9% (5.3) at 2 h; mean differences: 7.2 ±3.7% and 7.7 ±3.6%). These divergent responses were still evident the following morning (4.1 vs. -3.6%; difference: 8.8 ±6.5%). In conclusion, water balance is regained more slowly following exercise- than heat-induced hypohydration; presumably at least partly because some of BM deficit is attributable to glycogen metabolism and associated water release. Physiologically, this BM deficit may therefore not represent hypohydration. In contrast, plasma volume expansion was rapid and preceded fluid restoration, as found previously [4].