The effect of acute hypoglycaemia on renal function was examined in eight male patients with Type 1 diabetes who had normal urinary albumin excretion. Insulin was given as a bolus intravenous injection (0.125 U kg-1) and plasma glucose fell to a nadir of 1.6 (SE 0.2) mmol l-1, with all patients experiencing an acute autonomic reaction. Renal plasma flow fell from 674 (106) to 540 (198) ml min-1 during hypoglycaemia (p less than 0.01) and returned to 655 (181) ml min-1 (NS vs baseline). Glomerular filtration rate (GFR) declined from 143 (23) to 110 (36) ml min-1 during hypoglycaemia (p less than 0.02), rising to 150 (44) ml min-1 in the recovery period (NS vs baseline). The urinary flow rate and urinary albumin excretion rate both fell significantly in response to hypoglycaemia (10.6 (1.2) to 4.7 (1.1) ml min-1; p less than 0.002, and 46.2 (10.6) to 26.0 (10.5) micrograms min-1, respectively). Urinary dopamine excretion also declined, from 322 (37) to 211 (29) mumol min-1 (p less than 0.005) but sodium excretion was unchanged. Plasma adrenaline concentration (0.2 (0.03) to 1.7 (0.4) nmol l-1; p less than 0.01) and plasma renin activity (0.49 (0.13) to 1.08 (0.17) ng-Ang 1 l-1 h-1; p less than 0.01) increased during hypoglycaemia, but changes in plasma noradrenaline and angiotensin II levels did not attain significance. These acute changes in renal function, observed during hypoglycaemia in diabetic patients, may result from direct stimulation of the efferent sympathetic nerves to the kidney, complemented by the hormonal changes induced by hypoglycaemia.