The effect of NMDA on the motility of the rat portal vein was studied in an isolated preparation. NMDA induced a concentration-dependent (10(-7) - 10(-4) M) increase of the contraction frequency (maximum increase, 148+/-6% of control at NMDA 10(-4) M). The NMDA-induced excitatory response was prevented by the competitive NMDA receptor antagonists (+/-)-2-Amino-5-phosphonopentanoic acid (AP-5, 5x10(-4) M) or (RS)-3-(2-carboxypiperazine-4-yl) propyl-1-phosphonic acid (CPP, 10(-4) M). Tetrodotoxin (TTX, 10(-6) M) or atropine (10(-4) M) abolished the NMDA-induced increase of the portal vein motility and reversed the excitatory effect to a concentration-dependent inhibition (maximum inhibition, 52+/-8 and 29+/-7% of controls, respectively, at NMDA 10(-3) M). Removal of the endothelium abolished the NMDA-induced inhibitory response. Sodium nitroprusside concentration-dependently (10(-7) - 10(-5) M) inhibited the portal vein motility, while L-N(G)-nitro-arginine methyl ester (L-NAME, 10(-4) M) reversed the inhibitory effect of NMDA (in the presence of TTX), restoring the portal vein spontaneous activity to control values. These results show that NMDA modulates the portal vein motility in a biphasic manner: via indirect activation, through prejunctional NMDA receptors presumably located on intrinsic excitatory neuronal afferences, or via direct inhibition, through endothelial NMDA receptors activating the nitric oxide pathway. Overall these findings support the hypothesis of the existence of a peripheral glutamatergic innervation modulating the contractile activity of the rat portal vein. British Journal of Pharmacology (2000) 129, 156 - 162