The effect of glucose withdrawal upon mechanical activity and the redox level of pyridine nucleotides and flavoproteins was investigated in isolated rat portal veins under aerobic conditions. A decrease of extracellular glucose concentration correlated with contractile activity. During acetylcholine-induced tetanic contractions, NADH and flavoproteins were oxydized only in vessels supplied with glucose. The myogenic activity was restituted when glucose was added after withdrawal parallel with a reduction of NAD and flavoproteins. The results suggest that glucose, besides its effect on the energy metabolism, has an influence on the excitability of smooth muscle cells of rat portal veins.