Over 2 wk, beginning 3 wk postpartum, daily changes of feed and water intake and milk production and composition were monitored in 25 cows allotted randomly among four dietary treatment groups differing in sodium chloride, sodium bicarbonate, and ad libitum supplementation by salt block. The basic complete diet was 54% corn silage, 27% shelled corn, and 16% soybean meal, dry basis. The four dietary treatments were control, .42% chloride; low, .10% chloride; low plus salt block, complete ration same as low with ad libitum salt block consumption; and control plus .7% sodium bicarbonate. Differences were consistently significant only for electrolyte concentrations in blood serum, urine, and feces of cows fed the low diet. In blood serum, chloride declined from 96.4 +/- 3.5 to 83.4 +/- 4.5 meq/liter, and serum total carbon dioxide rose from 28.4 +/- 1.9 to 36.9 +/- 2.6 meq/liter. Anion gap increased from 19.0 +/- 2.9 to a high of 23.4 +/- 3.5 meq/liter in blood serum at day 11. Metabolic derangements of cows fed the low diet may be summarized as subclinical primary hypochloremic, secondary hypokalemic metabolic alkalosis.