Renal haemodynamics estimated using inulin- and para-aminohippuric acid-(PAH) clearances and segmental tubular handling of sodium as estimated using lithium clearance where studied in fourteen healthy men. Volume expansion was induced by a 2 h (25 ml kg-1) infusion of 0.9% sodium chloride (NaCl) load. Eight of the 14 subjects were rechallenged with a 2 h infusion of 5% glucose (25 ml kg-1). In addition, ten healthy subjects were investigated with inulin and PAH-clearances during water diuresis. When NaCl was infused glomerular filtration rate (GFR) decreased from 115 to 103 ml min-1 (p < 0.002) and fractional sodium excretion increased by 85%. The fall in GFR could be due to tubuloglomerular feedback as a result of inhibition of proximal tubular sodium reabsorption. The fall in GFR raises doubt about the usefulness of NaCl as an inert control infusion in metabolic studies. During glucose infusion blood glucose rose from 4.3 to 10.9 mmol l-1 with no significant change in GFR, but fractional sodium excretion was reduced by almost 40%. The etiology of the acute antinatriuretic effect of volume expansion with glucose infusion in healthy humans is not known but a blunted decrease in plasma renin activity and erythrocyte volume fraction in conjunction with a failure to mobilize renal dopamine and an increase in plasma levels of antinatriuretic factors such as insulin and norepinephrine are all factors that may contribute to the antinatriuretic effect of a glucose infusion.