SUMMARY1. The effects of adenosine and adenine nucleotides were studied on arterial vasculature in blood‐perfused arterial preparations of dogs which were isolated from the internal and external carotid arteries. Each compound was administered directly into the cannulated artery over a period of 4 s.2. In both arterial preparations, adenosine produced a dose‐related vasodilatation and was more potent than AMP. On the other hand, ATP produced a dose‐related vasoconstriction and was more potent tha