A bioassay method is described for the simultaneous identification and quantitation of small amounts of prostaglandins E2 and I2. Strips of rabbit coeliac and bovine coronary arteries were suspended within a common chamber surrounded by a heated water bath. The tissues were coupled individually to transducers to record their length. Using a superfusion flow rate of 2.0 ml/min the rabbit coeliac and bovine coronary arterial strips responded in a highly reproducible manner to threshold doses of 0.1 ng PGE2 and 1.0 ng PGI2, respectively. This technique is more sensitive than the conventional bioassay methods to quantify and differentiate between prostaglandins.