The influence of changes in the perivascular K+ and H+ concentrations upon adenosine-induced dilatations of pial arteries was investigated by localized perivascular application using micropuncture technique. Adenosine (10<sup>–9</sup>–10<sup>–3</sup> m) was dissolved in mock spinal fluids with varying bicarbonate and potassium concentrations (5 and 22 mM HCO<sub>3</sub><sup>–</sup>, and 6 and 10 mM K+). Concentration response curves revealed that adenosine induced the same change in pial arterial diameter when dissolved either in an inert (11 mM HCO<sub>3</sub><sup>–</sup>) or in a constrictory (22 mM HCO<sub>3</sub><sup>–</sup>) mock spinal fluid. However, adenosine, when dissolved in a more acidic or higher postassium-containing solution (5 mM HCO<sub>3</sub><sup>–</sup>, or 6 or 10 mM K+) produced a diminished dilatatory effect. It is concluded that, for a quantification of the individual contributions of several metabolic factors in the regulation of vascular resistance, it is necessary to consider their interactions. 1 A preliminary report of a part of this investigation was presented at the Symposium on ‘The Role of Ions in the Transmission of Signals from Tissue and Bloodto the Vascular Smooth Muscle Cells’, Tubingen, February 12–14, 1976 [publishedin Betz, 1976]. 2 Supported by the ‘Deutsche Forschungsgemeinschaft’.