A potent and selective 5-HT4 antagonist, with a pKi of 9.1 at 5-HT4 receptors in guinea pig striatal membranes and greater than 1000-fold selectivity over 5-HT1A, 2C, 3 and D1, D2, M1, M2, AT1, B1 and α1C receptors. The ketone group gives RS 39604 a relatively long half life; it is also orally active and so suitable for in vivo studies. Purity 98%. MW (observed): 590.56