Four primiparous Friesian cows in mid-lactation and housed in climate chambers were simultaneously exposed to three sequential climate treatments: 1, a three-week period in a thermoneutral environment (ambient temperature Ta 14-21 degrees C and relative humidity r.h. 60-70%); 2, a similar period during which they were exposed to Ta max. 38 degrees C/r.h. max. 80% for up to 7 h and Ta 14-21 degrees C/r.h. 60-70% for 17 h each day; and 3, a three-week period during which they were subjected once more to the thermoneutral conditions described for 1. Water and a complete feed were constantly available. Compared with water intake (drinking water + feed water) under thermoneutral conditions cows exposed to treatment 2 significantly increased their mean intake by 12.2%; in three cows, this involved a phase-shift of greater than 20% in drinking habits from day (hot) to night (cool) time. Water balance trials conducted at the mid-point of each treatment revealed that the mean losses of water via urine, faeces, milk, sweat and saliva as a percentage of water intake changed significantly resulting in a net gain (retention) of body water. An accompanying significant increase in live weight despite a 9.1% decrease in DM intake during treatment 2 confirmed the water retention results. On return to thermoneutral conditions (3), the cows exhibited a marked weight loss and a significant increase in urinary water excretion over treatment 1 and 2 values, signifying that a large proportion of the water retained during 2 was of extracellular origin.(ABSTRACT TRUNCATED AT 250 WORDS)