Choline Magnesium Trisalicylate (Trilisate), in therapeutic concentrations of 5, 10, 15 and 30 mg/100 ml, did not significantly affect production of prostacyclin-like (PGI2) substance by rat aortic tissue in vitro. The ED50 for inhibition of aorta PGI2-like substance production by Trilisate was 1,200 mg/100 ml. This is approximately 40 times the maximum therapeutic blood concentration achieved in humans. Choline or Magnesium salicylate produced slight but insignificant inhibition of PGI2-like substance production by rat aortic tissue in vitro. The ED50 for ibuprofen (Motrin) for inhibition of PGI2-like production of rat aortic rings was 0.65-0.92 mg/100 ml. Injection of Choline Magnesium Trisalicylate into rats (124, 250, 500 mg/kg I.P.) did not affect the normal production of PGI2-like substance of aortic tissue obtained one hour after in vivo treatment. These results suggest this anti-inflammatory salicylate does not adversely affect PGI2-like production by blood vessels, in concentrations associated with therapeutic effects in man.