The release of prostaglandinlike substances from canine pial arteries that was induced by exposure of the pial arteries to red blood cell hemolysate was estimated by using a superfusion technique for prostaglandin bioassay. The assay organs used were strips of rat stomach for prostaglandin E2- or prostaglandin F2 alpha-like substances, strips of dog coronary artery for prostaglandin I2-like substance, and strips of dog ileum for prostaglandin F2 alpha-like substance. The substances released from the canine pial arteries induced contractions in rat stomach strips and relaxations in canine coronary arterial strips, whereas they did not induce any response in canine ileal strips. The equivalent prostaglandin E2 doses for the contractions of the rat stomach strips and the equivalent prostaglandin I2 doses for the relaxations of the canine coronary arterial strips were 125.2 +/- 19.4 (n = 8) and 59.5 +/- 16.4 (n = 6) pmol/g wet wt +/- SEM, respectively.