The present study was designed to investigate the influence of the endothelium and that of the L-arginine pathway on the contractile responses of isolated human cerebral arteries to electrical field stimulation (EFS) and norepinephrine. Rings of human middle cerebral artery were obtained during autopsy of 19 patients who had died 3–8 h before. EFS (1–8 Hz) induced frequency-dependent contractions that were abolished by tetrodotoxin, prazosin, and guanethidine (all at 10^-6 M). The increases in tension were of greater magnitude in arteries denuded of endothelium. N^G-monomethyl L-arginine (L-NMMA 10^-4 M) potentiated the contractile response to EFS in artery rings with endothelium but did not influence responses of endothelium-denuded arteries. L-arginine (10^-4 M) reversed the potentiating effects of L-NMMA on EFS-induced contractions. Norepinephrine induced concentration-dependent contractions, which were similar in arteries with and without endothelium or in arteries treated with L-NMMA. Indomethacin (3 × 10⁻⁶ M) had no significant effect on the contractile response to EFS or on the inhibition by L-NMMA of acetylcholine-induced relaxation. These results suggest that the contractile response of human cerebral arteries to EFS is modulated by nitric oxide mainly derived from endothelial cells; although adrenergic nerves appear to be responsible for the contraction, the transmitter involved in the release of nitric oxide does not appear to be norepinephrine. The effects of L-NMMA in this preparation appear to be due to inhibition of nitric oxide formation rather than caused by cyclooxygenase activation.