SUMMARYTwo patients with severe ovarian hyperstimulation syndrome are described. Increased plasma concentrations of immunoradlometrically determined total renin are shown, together with greatly increased plasma levels of active renin and aldosterone. These very high values for total renin, renin activity and aldosterone were not suppressed when extracellular compartments were greatly expanded; the values subsequently declined to normal levels, despite the use of diuretics. This suggested that the renin was of non‐renal origin since its production was apparently unaffected by influences which control juxtaglomerular secretion. The high concentrations of the rennin—angiotensin—aldosterone system suggest that it contributes to the genesis of the ovarian hyperstimulation sy