Abstract: The aggregation of platelets on the surface of a foreign body exposed to the flowing blood of a living being (such as plastic tubing, a balloon or the end of a catheter surgically inserted in a blood vessel, a stent implanted therein or synthetic grafts, which surface normal promotes such platelet aggregation to form a coating firmly affixed to that surface which would restrict the flow of blood past that surface or to form a blood clot detachable from that surface), is inhibited by a gas permeable coating on the surface of a physiologically acceptable polymer as which contains dissolved or dispersed therein a nitrosyl-containing organometallic compound, such as sodium nitroprusside, which is protected from diffusion from the coating and from direct contact with the blood and which slowly decomposes at the body temperature within the coating and in so doing releases a platelet aggregation-inhibiting amount of nitric oxide which diffuses from the coating during the period when platelet aggregation by the surfa