1. In four patients with nephrotic syndrome indomethacin not only reduced proteinuria but also inhibited the natriuretic effect of high doses of frusemide.
2. The inhibition of natriuresis by indomethacin could not be antagonized by albumin infusions.
3. Only the combined use of spironolactone and frusemide induced a natriuresis during indomethacin treatment. Spironolactone alone was ineffective.
4. It is suggested that inhibition of prostaglandin synthesis by indomethacin, in the presence of a stimulated renin-angiotensin system and hyperaldosteronism, may cause this strong tendency to sodium retention.