The experimental drugs butanserin (R 53393), ritanserin (R 55667), R 56413, flufylline (Sgd 195/78) and fluprofylline (Sgd 144/80) were evaluated with respect to their antagonism at postjunctional alpha 1- and alpha 2-adrenoceptors and 5-HT2-receptors in pithed rats. Moreover, affinity for [3H]mianserin, [3H]prazosin and [3H]yohimbine binding sites was assessed using rat brain preparations. In all experiments ketanserin was taken as a reference compound. It is concluded that of the compounds investigated butanserin is the most potent and selective alpha 1-adrenoceptor antagonist, whereas ritanserin was found to be a potent and selective 5-HT2-antagonist. Of the other compounds, fluprofylline was a very selective though not very potent alpha 1-adrenoceptor antagonist. The other compounds were less active and less selective in this respect.