To study the onset of tissue ablation by cw laser irradiation, aortic and myocardial bovine tissue samples were exposed in air to a 1 mm diameter laser beam of either 15 W Nd-YAG or 3.5 W Argon. Exposure times varied from 0.1 to 30s. The surface of the tissue was filmed with a resolution of 0.01 mm and recorded on video with time coding. A photodiode positioned underneath the tissue samples measured forward transmitted light. After exposure the tissue was processed for histology. The observed phenomena were divided in three phases: Phase A: The tissue surface slightly discolored while the transmission signal remained constant or slightly diminished. Phase B: Simultaneous with a 'pop' sound, the surface rose and the transmission dropped abruptly to about 50% and remained at this level for several seconds during vaporization of tissue water. Histology showed ruptured layers and multiple vacuoles beneath the surface. Phase C: In the middle of the beam a spot of carbonized tissue was formed which grew concentrically from the center while a crater was formed.