Catheter-guided laser myoplasty in a closed ventricle has been advocated for the treatment of hypertrophic cardiomyopathy, ablation of arrhythmogenic foci, and transmyocardial laser revascularization of ungraftable regions of ischemic myocardium. The purpose of this study was to evaluate the nature of particulate debris and photoproducts generated in vivo. Accordingly, cardiopulmonary bypass was established in four dogs without active cooling and an apical left ventricular vent was placed. In two dogs, a laser catheter was inserted into the cardioplegia-arrested left ventricle through the left atrial appendage and across the mitral valve. In the other two dogs the laser catheter was inserted into a perfused, fibrillating left ventricle retrograde across the aortic valve. Myocardial ablation of the ventricular septum was performed using continuous argon-ion laser irradiation (8-10 watts, 4-11 minutes) and blood samples were drawn through the ventricular vent. The final ablation in each dog was done during a separate ventricular flush with a chemical "spin trap" capable of detecting free radicals. Analysis of ventricular effluent revealed levels of carboxyhemoglobin and hydrogen cyanide which were well below accepted clinical toxic limits. Microparticulate debris (2-300 femtoliter) was not detected. Evidence of trace free radical generation was suggested in one of four dogs. In conclusion, closed ventricular argon laser myoplasty did not generate clinically significant levels of the toxic photoproducts we measured, or microparticulate debris. These in vivo findings suggest that the clinical application of catheter-based laser myoplasty will not be limited by the generation of these toxic photoproducts or microembolic debris.