The NANC inhibitory innervation of the longitudinal muscle of the rabbit portal vein has been examined. Neurogenic relaxations were partially inhibited by the P2-purinoceptor antagonist, suramin. Addition of the NO-synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) also significantly reduced responses to electrical stimulation and the addition of L-arginine reversed this effect. A combination of both suramin and L-NAME abolished the neurogenic relaxation. A maximum relaxation of the vein was evoked by sodium nitroprusside which was not affected by L-NAME or suramin. Histochemical staining demonstrated the presence of NADPH-diaphorase containing nerves between the longitudinal and circular muscle coats of the media and also in the adventitia. It is concluded that both ATP and NO are inhibitory neurotransmitters in the NANC nerves of the rabbit portal vein.