A new potential antihypertensive drug, irindalone (Lu 21 -098;( +)-(1 R,3S)-1-[2-[4-[3-(pf luoropheny1)-1 -indanyl]-1 -piperazinyl]ethyl]-2-imidazolidinone), has been characterized by a series of in vitro methods. lrindalone competitively inhibits peripheral and central serotonin;! (5-HT2) receptors as measured by 5-HT-induced contractions of rabbit pulmonary artery and receptor binding methodology with 3H-ketanserin as a ligand. The inhibitory effect on 5-HT receptors is confirmed using other organs in vitro (rat stomach fundus, rat jugular vein, guinea pig trachea). lrindalone has lower affinity for a,-adrenoceptors and histamine-H, receptors and only negligible affinity for a,-adrenoceptors and muscarine cholinergic receptors. No inhibition of the uptake of noradrenaline (NA), dopamine (DA), or 5-HT is seen. The neurochemical profile resembles that of ketanserin, which has slightly higher affinity for 5-HT2 receptors. In the rat fundus, irindalone inhibits 5-HT-induced contractions, differing in this respect from ketanserin and cinanserin. lrindalone clearly deviates from the neuroleptics haloperidol and teftudazine, and from prazosin with which irindalone has been compared. The (-)enantiomer of irindalone, Lu 21 -099, is considerably weaker than irindalone in the different test systems. Due to the strong blockade of 5-HT2 receptors and the somewhat weaker blockade of a-adrenoceptors, irindalone is expected to lower blood pressure potently and thus be a candidate for the treatment of hypertension.