Lactation is a physiologically demanding period in mink production, during which kit and dam losses may occur. Ambient temperature and quantitative water supply are thought to affect animal performance and well-being, but conclusive data in the literature are sparse. Therefore, effects of ambient temperature (Ta; low, about 5° medium, about 15° high, average 20-25°) and water supply (ad libitum (N), or 10 % extra supplementation in the food (E)) were investigated regarding effects on quantitative water intake and excretion, urine osmolality and solute excretion, and urinary cortisol and catecholamines as stress indicators in an experiment with twelve lactating mink with litters of three to seven kits in three consecutive periods, lasting 3, 3 and 2 d respectively. Kit ages ranged from 15 to 20 d at the end of the experiment. Water requirement for milk production (factorial calculations) and water available for evaporation (balance component) were estimated. Period, and hence mainly Ta, had a significant influence on intake of metabolizable energy, quantitative water intake and excretion, but there was less effect of water supply. The total water intake and excretion were very high in relation to the weight of the animals as an effect of lactation. Water intake and excretion, and urinary Na excretion, seemed to be less accurately regulated compared with corresponding functions in non-lactating animals. Rectal temperature increased with increasing Ta, possibly as a means of decreasing evaporative water loss. Water output in milk was estimated to increase from 118 g/d at low Ta to 134 g/d at high Ta. The amounts of water available for evaporation were estimated to be 42, 58 and 69 g/kg0·75 at low, medium and high Ta. Cortisol data did not indicate that the animals experienced negative stress. It was concluded that prolonged periods of high Ta may be hazardous for lactating mink because of decreased intake of metabolizable energy resulting in energy deficit and excessive mobilization of body reserves simultaneously as the requirement for intake of water increases considerably.