Intravital microscopy of rabbit tenuissimus muscle was used for studies of endogenous nitric oxide as a microvascular regulatorin vivo.Derivatives of were administered in order to modulate the formation of nitric oxide from L‐arginine. Nw‐nitro‐L‐arginine methylester (L‐NAME) (1‐100 mg kg‐1i.v.) dose‐dependently reduced microvascular diameters. A concomitant blood pressure increase and a decrease in heart rate was observed. The blood pressure increase induced by L‐NAME (30 mg kg‐1) was reversed by L‐arginine (1 g kg‐1) but not D‐arginine. Vasodilation in response to topical acetylcholine (0.03‐3 μM) was significantly inhibited by L‐NAME (30 mg kg‐l), whereas vasodilation by sodium nitroprusside (300 nM) was not affected. Vasomotor nerveinduced vasodilatation, induced by stimulation of the tenuissimus nerve after neuromuscular blockade by pancuronium in animals pretreated with guanethidine, was significantly attenuated by L‐NAME, an effect also reversed by L‐arginine. The vasodilatation in response to active contractions of the muscle induced by motor nerve stimulation as well as the vasodilator response elicited by graded perfusion pressure reductions were unaffected by L‐NAME or NG‐monomethyl‐L‐arginine (L‐NMMA, 10‐4M) administered topically.Our results indicate that endogenous nitric oxide formed from L‐arginine is a modulator of microvascular tonein vivo.Furthermore, the results suggest that endogenous nitric oxide is required for vasomotor nerve‐induced vasodilatation, whereas it does not appear to play a role in myogenic va