• NO concentration was measured in the periendothelial area of the femoral artery by Malinski’s porphyrinic • NO sensor in seven anaesthetized dogs. The basal concentration was 154.2 ±5.6 nM and two-minute intraarterial infusions of acetylcholine (3-4 /tg/ml/min) or bradykinin (30 - 40 ng/ml/min) increased this value significantly to 204.3±16.4 and 266.5±16.4 nM (P<0.01), respectively. Inhibition of »NO synthase by L-NAME (50 mg/kg) declined the basal • NO concentration only to 137.2±3.3 nM (PcO.Ol). Subsequent administration of acetylcholine and bradykinin attenuated significantly the increase in • NO concentration. Surprisingly, both agonists still induced a significant increase of *NO concentration by 125.3±8.3 and 156.6±26.9 nM, respectively (PcO.Ol). One of the possible explanations may be that besides arginine-citrulline plus the • NO pathway other sources of • NO could be involved in the high level of • NO after • NO synthase blockade by L-NAME.