Prostaglandins modulate the effects of vasoactive hormones by attenuating the renal actions of the renin-angiotensin system and contributing to and, perhaps, mediating some of those of the kallikrein-kinin system. A prostaglandin mechanism participates in the regulation of renin and erythropoietin release. When renal function is compromised, the circulation to the kidney is sustained by a major prostaglandin component withdrawal of which results in significant hemodynamic effects, particularly reduction of blood flow to the inner cortex and medulla.