Eight hypertensive children with acute post-streptococcal glomerulonephritis were given intravenous frusemide, 2 mg/kg, and the results compared with 8 similar cases not given the diuretic. Mean urine flow increased from 0.24 ml/min/m2 before frusemide to 3.63 ml/min/m2 in the 6 hours afterwards and was still 0.72 ml/min/m2 48 hours later. In contrast mean urine flow remained unchanged over 48 hours in those not given frusemide. Despite similar initial blood pressures the duration of hypertension was much shorter (mean 4.7 days) after frusemide than in the controls (mean 11.0 days) and the edema-free weight was achieved more rapidly (6.8 days compared with 13.9 days). Plasma renin activity (PRA) did not rise after frusemide in the children with acute nephritis. This was in contrast to the rapid rise seen in normal humans thus indicating a dissociation between the diuretic and renin-releasing activities of frusemide in acute nephritis. Seven children with the hemolytic-uremic syndrome or acute tubular necrosis showed no significant change in either urine flow or PRA after frusemide. Frusemide is therefore effective treatment for both hypertension and oliguria in acute nephritis. Failure of PRA to rise indicates that renin release mechanisms are abnormal in renal failure and that PRA levels need to be interpreted with caution in this condition.