We have investigated the experimental consequences of two picosecond infrared lasers, both tuned to 6.45micrometers and focused on ocular tissue. The exposure conditions were comparable, other than pulse repetition rate, where an optical parametric oscillator/amplifier laser (OPA) system operates at a kilohertz and the Mark-III FEL at 3 gigahertz. In both cases, the peak intensity was near 2x1014 W/m2 and the total delivered energy was approximately 125 mJ. The Mark-III consistently ablates tissue, while the OPA fails to ablate or to damage corneal tissue. In particular, there is no experimental evidence for protein denaturation due to OPA irradiation. WE account for these observations in terms of a theoretical model based on thermal diffusion and threshold conditions for superheating and chemical kinetics. We comment on the relevance of tissue geometry.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.