Injection into the superior colliculus of anaesthetised rats of the nitric oxide (NO) synthase inhibitorN ω -nitro-l-arginine methyl ester (l-NAME; 1 μmol), but not its inactive enantiomer N ω -nitro-d-arginine methyl ester (d-NAME; 1 μmol), significantly (P < 0.01) increased the mean arterial blood pressure. Injection to the superior colliculus of l-arginine (l-Arg; 1 μmol), the substrate for NO synthase, significantly (P < 0.01) lessened the pressor effect of l-NAME, while d-arginine (d-Arg; 1 μmol) did not affect it. l-Arg (7.5 μmol), but not d-Arg (7.5 μmol) administered at the peak of the pressor response to l-NAME (1 μmol) also partially reversed this pressor response (P < 0.05). These data would suggest that endogenously produced NO acts within the superior colliculus to modulate the arterial blood pressure.