PGSI cause renal vasoconstriction in furosemide- and low-salt-treated dogs, but not in untreated normotensive dogs, and this effect is closely related to their control PRA. PGSI cause a lesser degree of renal vasoconstriction in 2-kidney 1-clip Goldblatt hypertensive dogs which does not correlate with their control PRA. PGSI abolish PG modulation of the renin-angiotensin system, and this effect appears to account for the increased renal vascular tone induced in furosemide- and low-salt-treated dogs.