Responses of the aorta and portal veins isolated from rats to aqueous extract of plantain (Musa paradisiaca) were studied. The extract produced concentration-dependent relaxation in both noradrenaline- and KCl-contracted aortic rings. The maximum relaxation in noradrenaline-contracted rings was 52.49 +/- 6.63% and in KCl-contracted rings was 77.51 +/- 2.52% of the initial tensions developed in response to the contractile agents. The extract also produced significant (P < 0.01) inhibition of the maximum aortic contractile response to noradrenaline and completely abolished the spontaneous contractions of the portal veins. Serotonin (5-hydroxytryptamine), unlike the extract, produced contraction rather than relaxation of the aortic rings. The results show a non-specific relaxing and inhibiting effect of plantain extract on aortic and portal smooth muscles, an effect that is not attributable to its abundant serotonin content.