Tissue nonlinear spectroscopy has been investigated as a potential method for monitoring the laser photo-heating and laser ablation of biological tissues samples. The nonlinear optical phenomenon of the second harmonic generation (SHG) for femtosecond-picosecond duration laser beam pulses passing through collagen containing a layer of biotissue was used to monitor the photothermal processes induced by laser irradiation. The samples of biotissues were photo-heated by CW Nd:YAG laser. A picosecond Nd:YAG beam with 1064 nm and pulse duration tau(imp) = 33 ps was used for probing. After the laser heating of fasciae and tendon samples, the SHG radiation decreased six times in the area of irradiation. The laser ablation of skin samples was performed with a CW Nd:YAG laser at the power of 2 W. As a result of laser ablation the SHG nonlinear signal increased by a factor of 2. The results demonstrate, that the SHG nonlinear phenomenon enables real-time monitoring of photo-heating and ablation processes of biological tissue in novel laser surgical procedures. (c) 2005 Elsevier B.V. All rights reserved.