In the present work we studied the relaxant effect of two aporphinic alkaloids isolated from the roots barks of Cissampelos glaberrima St. Hil. (Menispermaceae), (+)-cissaglaberrimine (CGE; a tertiary alkaloid) and (+)-trilobinine (TBE; a quaternary alkaloid). In guinea-pig tracheal preparations, CGE and TBE reduced the spontaneous tone and inhibited the contractions induced by carbachol and histamine. TBE was 6 times more potent than CGE in reducing the spontaneous tone and it was also 1.5 times more potent in antagonising the effects of carbachol and histamine. The inhibitory effect of CGE in contractions induced by histamine was not attenuated in the presence of timolol (10 microM). However, in the same experimental conditions, timolol almost completely abolished the inhibitory effect of TBE. These results showed that the relaxations induced by TBE were dependent on the activation of beta 2-adrenoceptors, while the mechanism involved in relaxations induced by CGE remains to be determined.