The urinary excretion of albumin and beta 2-microglobulin in response to two different types of oral water load was studied in 18 healthy subjects. Both acute water loading (1 litre of tap water given over 10 min) and chronic water loading (250 ml of water given every 20 min for the 4-h duration of the test) produced a short-lived but significant increase in the urinary excretion of albumin, but not in beta 2-microglobulin. Albumin excretion returned promptly to basal values in spite of high and sustained urine flow. The decrease in haematocrit, plasma osmolality, plasma sodium concentration and pulse rate and the increase in creatinine clearance suggest that the elevation in albumin excretion might be the result of a transient volume expansion associated with an increase in glomerular filtration rate and in filtration of albumin. However, a washout of proteins from the tubular lumen with preferential reabsorption of small molecular weight proteins (i.e. beta 2-microglobulin) during high tubular flow cannot be excluded. Studies investigating changes in urinary albumin excretion should be performed after baseline conditions of stable urinary excretion of albumin have been established.