KTP (potassium titanyl phosphate) laser was successfully applied to neuroendoscopic procedures of intra-ventricular lesions: fenestration of cyst-wall or the floor of the third ventricle (third ventriculostomy) in 27 patients, and vaporization of tumor or adhesive granulation tissue (in case of shunt-tube removal) in 5 patients. Prior to clinical use of KTP laser, temperature change in the ventricle caused by KTP laser irradiation was investigated using a plastic model of the ventricle. With KTP laser output up to 15 watts in 1 sec pulse mode, temperature increase in the ventricle model was within 5 degree. Continuous irrigation with saline effectively protected the irradiation-induced temperature increase. However, over 10 degree increase in temperature was measured, when the output of 20 watts 1 sec pulse mode was employed in the air. These results indicate that optimal conditions of KTP laser for clinical use would be 2-5 watts for the fenestration and 5-15 watts for the vaporization in 1 sec pulse mode with continuous irrigation of the ventricle by lactate Ringer's solution.