The effects of a strenuous marathon run of 42.2 kilometers on the presence of prostaglandins in blood was investigated in 24 healthy males and females. Testosterone, FSH, LH, epinephrine, norepinephrine, dopamine, T4 and T3 were also measured. With the exercise of the marathon run, a significant elevation of plasma PGE2, PGF2 alpha and 6 keto PGF1 alpha was observed in the runners following the race. Significant increases were also noted in blood catecholamine, dopamine and testosterone levels while a significant decline in the gonadotropins, FSH and LH was observed. No changes in thyroxine or tri-iodothyronine were noted.