A purified diet containing 10% linseed oil as the fat source was fed to rats over a 56-day period. After the 56th day the rats were fed the same basal diet but containing 10% corn oil in place of the linseed oil. Rats were killed and blood and liver samples were taken from four to six rats on 14 days of the feeding trial. Serum and liver fatty acid profiles were determined. The platelet prostaglandin E2 (PGE2) released in serum as a result of blood coagulation for 1 h at 37 degrees C was determined. Liver homogenates were incubated and PGF2 alpha synthesizing capacity was assayed. Both serum and liver fatty acid profiles reflected the high linolenic content of the linseed oil. There was a progressive increase in fatty acids of the omega 3 series and a decrease in the omega 6 series. Notably the PG-2 series precursor, arachidonic acid (20:4 omega 6) was decreased and the precursor of the PG-3 series timnodonic acid (20:5 omega 3) was increased. These changes were reversed when corn oil was fed. PGE2 content of serum and PGF2 alpha synthesis by liver homogenates progressively decreased as the 20:4 omega 6 levels fell and the 20:5 omega 3 levels increased. PG synthesis was quickly increased in both when corn oil diets were fed. This study has implications for dietary manipulation of PG synthesis by blood components and may provide a basis for manipulation of PG synthesis in systems in which they are regulatory.