Bitches maintained on a low Na intake, were given doses of saline (0.125 mol.litre(-1) NaCl, 0.025 mol.litre(-1) NaHCO3, 0.004 mol.litre(-1) KCl) by stomach tube. Doses of 100 and 200 cm3 produced only minor increases in Na excretion; after 300 cm3, Na excretion rose from about 2 to about 60 micromol.min(-1). Plasma protein fell by 1.8 litre(-1) for each 100 cm3 of saline retained. Within normal ranges of Na excretion there is a threshold of plasma protein concentration above which Na is retained and below which Na is excreted. Changes in exogenous creatinine clearance were measured allowing calculation of the filtered load of Na, which shows that the absolute tubular reabsorption of Na and water is increased in volume expansion by isotonic saline. Meat produced large increase in glomerular filtration rate without much increase in Na excretion and mechanisms are discussed by which Na reabsorptin is more effective after meat than after doses of saline. Creatinine cleaerance increased by 0.67 cm3.min(-1) for each fall of 1 g.litre(-1) in plasma protein; this is predicted by a theory that the glomerular capillary blood pressure is 9.3 kPa (70 mmHg) rather than 6.7 kPa (50 mmHg).