In the present study, two 6-(fluorobenzoyl)-3-piperazinomethyl-2benzothiazoline ne derivatives were synthesized and their relaxant effects on isolated rabbit corpus cavernosum investigated. Compounds Y-16 and Y-21 can alter the ability of corpus cavernosum smooth muscle to contract. Strips of rabbit corpus cavernosum smooth muscle were mounted in isolated tissue baths for measurement of isometric contractile force. Compounds (10(-6)-10(-3) M) did not cause contraction but induced relaxation in precontracted corpus cavernosum smooth muscle. Neither N-nitro-L-arginine methylester (L-NAME) nor indomethacin affected the relaxant effect of these compounds. Glibenclamide and tetraethylammonium chloride (TEA) also did not influence the relaxation induced by the compounds. In conclusion, in isolated rabbit corpus cavenosum, Y16 and Y21 have a relaxant potency equal or superior to known vasoactive agents. Further investigations are needed to show the importance of these effects for the diagnosis and treatment of erectile dysfunction.