Continuous electrical field stimulation (EFS) elicited a sustained contraction and significantly increased the EC50 value of norepinephrine (NE), shifting the concentration-response curve for NE to the right. Tetrodotoxin significantly reduced the continuous EFS-evoked increases in basal tone and produced further dextral shift in the concentration-response curve for NE. N-methylhydroxylamine and NG-monomethyl-L-arginine (L-NMMA) attenuated the "dual" effects of continuous EFS on NE-induced contractions. L-Arginine partially reversed the inhibitory effect of L-NMMA. The results of the present study suggest that continuous EFS causes simultaneous release of both excitatory and inhibitory neurotransmitters, and the interaction (or functional antagonism) between the inhibitory neurotransmitter (endogenous nitric oxide, NO) and the excitatory neurotransmitter (endogenous NE), as well as exogenous NE, may occur at postjunctional site(s).