Vasoreactivity of laser-treated vessels was investigated in two different experimental conditions. The canine left circumflex coronary artery (LCx) was lased under perfusion with Krebs-bicarbonate buffer by means of a thermal laser (hottip probe, HT) at 7W for 6 seconds and an argon laser beam through a 300μm optical fiber at 3W (tip power) for 1 second at 12 spots. A nontreated segment of the LCx served as a control. Two 3-mm long segments were obtained from the treated segment: one to measure the results of potassium (K) induced contraction, and another 3, 4 diaminopyridine (DAP; K channel inhibitor) induced contraction. In 11 instances, coronary angiography of the perfused artery showed less than 50% stenosis after laser treatment. The segments were then mounted isometrically with 1g tension in Krebs-bicarbonate buffer. Contraction was induced either with 30mM KCI or 10^-2M DAP and expressed as developed tension (gram; g). KCI induced vasocontraction of 4.15±0.93g in the control, 0.33±0.71g in laser irradiated segments (P<0.0001vs control), and 0.02±0.06g in thermally-treated segments (P<0.0001 vs control). DAP induced vasocontraction of 5.21±1.32g in the control, 0.39±0.83g in laser irradiated segments (P<0.0001 vs control), and 0.07±0.13g in thermally treated segments (P<0.0001 vs control). In 4 instances, more than 5O% stenosis remained and additional balloon dilatation reduced the stenosis to less than 50%. The lesions also showed reduced vasoreactivity. In vivo thermal angioplasty resulted in reduced vasoreactivity compared to control in 4 anesthetized dogs. Thus, laser and thermal angioplasty reduced vasoreactivity induced by either KCI or 3, 4 DAP. Neither acetylcholine at 10^-6M nor papaverine at 10^-4M was able to induce relaxation of treated segments. In conclusion, 1) the lased coronary artery loses its vasoreactivity to either a constrictive or relaxing agent, 2) although stenosis may be produced by laser energy, additional balloon dilatation can reduce residual stenosis, and 3) laser thermal or argon laser angioplasty may prevent severe coronary spasm.