New serotonine 2(5-HT₂) antagonists with a monocyclic or bicyclic 2, 4(1H, 3H)-pyrimidinedione have been prepared and their activities evaluated. In a series of monocyclic compounds, 1-substituted 5-phenyl-2, 4(1H, 3H)-pyrimidinedione 14 showed potent in vitro activity, and the corresponding 3-substituted 5-phenyl and 6-phenyl derivatives 3, 8 and 20a also showed moderate activity. In the bicyclic compounds, 3-substituted 5, 6, 7, 8-tetrahydro-2, 4(1H, 3H)-quinazolinedione 33 exhibited the most potent activity among the compounds prepared in this paper. The in vivo antagonist activity of 33 was comparable of that of ketanserin, a typical peripheral 5-HT₂ antagonist.