Studies have examined adding protein to carbohydrate-electrolyte rehydration drinks, but the effects of protein in isolation remain unknown. Ten subjects completed two trials in which they were dehydrated (~2 % of pre-exercise body mass) by intermittent cycling in the heat. Subjects then rehydrated (150 % total mass loss) over 1 h with mineral water (W) or mineral water plus 20 g L(-1) whey protein isolate (WP) and remained in the laboratory for a further 4 h. Blood and urine samples were provided pre-exercise, post-exercise, post-rehydration and every hour thereafter. From blood samples, serum osmolality, change in plasma volume and plasma albumin content was determined, whilst the volume and osmolality of urine samples were determined. There was no difference between trials for total urine volume [W: 1,234 (358) mL; WP: 1,306 (268) mL; P = 0.409], drink retention [W: 40 (14) %; WP: 37 (14) %; P = 0.322] or net fluid balance [W: -605 (318) mL; WP: -660 (274) mL; P = 0.792] 4-h post-rehydration. Plasma volume was greater 3 and 4 h post-drinking during WP, and plasma albumin content relative to pre-exercise was increased 1-4 h post-drinking in WP only. These results suggest that addition of 20 g L(-1) whey protein isolate neither enhances nor inhibits post-exercise rehydration, when a volume equivalent to 150 % of sweat losses is ingested in 1 h. As post-exercise nutritional requirements are multifactorial (rehydration, glycogen resynthesis, myofibrillar/mitochondrial protein synthesis), these data demonstrate that when post-exercise protein intake might benefit recovery or adaptation, this can be achieved without compromising rehydration.