The present study has investigated the effect of prostaglandins and PAF on the contractility of the bovine ciliary muscle, a tissue involved in the control of aqueous outflow. The results show that the prostaglandins tested (PGI2, its stable analogue Iloprost, PGE2, PGE1, and PGF2 alpha) as well as PAF, were able to contract the ciliary muscle, although with different potencies and efficacies. PGI2 and Iloprost displayed parallel dose-effect curves with upper plateaus that did not differ significantly; however, PGI2 was slightly more potent than Iloprost. This is at variance with what is observed at the level of the platelet prostacyclin receptor. PGF2 alpha was equipotent with the PGEs tested, with a maximal effect not different from either PGI2 or PGEs. PAF was the most efficacious of the compounds tested.