Groups of nine or ten cows were assigned, after calving, to treatments in which they were (i) machine milked three times daily (M3), (ii) machine milked six times daily (M6) or (iii) suckled three times daily in addition to being machine milked three times daily (S). Treatments were administered during the first 6 weeks postpartum. On one day, at weeks 1 and 6 postpartum, blood samples were collected from all cows at 30-min intervals between 06.00 and 13.00 h and these were analysed for plasma osmolality and plasma concentrations of Na+, K+ and Cl−. Milk yield was significantly higher in suckled cows than in cows milked six times daily, but significantly lower in cows milked three times daily. In cows milked six times daily, and to a greater extent in suckled cows, there was a reduction in plasma osmolality and monovalent ion concentrations (Na+, K+ and Cl−), which could increase the susceptibility of the cows to water intoxication. Moreover, suckling or milking the cows six times daily was associated with increased fluctuations in plasma osmolality and plasma Cl− concentrations. The decrease in plasma osmolality and ion concentration and the increased variation in plasma osmolality and Cl− were probably related to increased water intake and may be indicative of a severe challenge to homoeostasis regulation.