The level of cyclic adenosine monophosphate (cAMP) in human platelets is known to be an important regulator of platelet function. The polyunsaturated fatty acids (PUFA) dihomo-gamma-linolenic acid (DHLA), and eicosapentaenoic acid (EPA), precursors of the prostaglandin (PG) 1 and 3 series respectively, were studied for their ability to stimulate platelet cAMP and/or PGE 1 levels, and to inhibit platelet aggregation (PAg). Incubation of washed platelets (1 10 8 /ml) with 125 μM DHLA increased intraplatelet levels of PGE 1 from 197±7 to 1622±9.7 picograms/10 8 , cAMP from 3±0.8 to 31±1.9 picomoles/10 8 , and inhibited collagen-induced PAg. Addition of 1 μmole of xanthine per unit of xanthine oxidase (a superoxide radical generating system) to the incubating medium potentiated the effects of both fatty acids, whereas 240 μM Hydrogen Peroxide (H 2 O 2 ) inhibited these effects. These results suggest that: (1) DHLA may be more effective in inhibiting PAg than EPA, which has been reported to reduce the incidence of coronary diseases in some human populations; (2) That superoxide radical may activate the platelet cyclooxygenase system to increase lipid peroxidation of these PUFA prostanoid precursors and may result in the inhibition of PAg, whereas H 2 O 2 may have an opposite effect.