PIP: Prostaglandins are naturally occurring compounds which are easily separable from other biologically active substances because of its acidic lipid nature. They are divided into 4 series, the A, B, E, and Falpha series, which differ in the structure of the characteristic 5-membered ring. Prostaglandins play a role in the following reproductive functions: 1) conception; 2) luteolysis; 3) menstruation; and 4) parturition. It has also been proposed that Prostaglandin A may be the natriuretic hormone, the circulating hormone which controls sodium reabsorption by the kidney. The experiments of Lee and Ferguson in which prostaglandins inhibited PAH uptake by rabbit renal cortex slices in vitro support this view. Prostaglandins are also implicated 1) in the fluid transfer in the gut; 2) as causative agents of diarrhea that accompanies medullary carcinoma of the thyroid or neural crest tumors; 3) in reducing blood pressure in humans with essential hypertension; 4) in fatty acid metabolism, including lipolysis; and 5) as mediators of the inflammatory response. Further research in prostaglandins will establish the validity of the proposed physiologic or pathologic functions of prostaglandins.