Low level laser radiation therapy is effective in a number of clinical situations (e,g. pain relief, wound healing, sports medicine), but the photobiological basis of this therapy is not well-understood, Since both visible and infrared radiations have been shown to be beneficial in such therapies, and since these two radiations differ dramatically in their photochemical and photophysical properties, how can they produce similar results clinically? I propose a modification of the model of Karu¹ to explain this. In her model. visible light produces photochemical changes in photoreceptors in the mitochondria, which alter metabolism, which leads to signal transduction to other parts of the cell (including membranes), which finally leads to the photoresponse (i.e, biostimulation). While visible light probably starts the cascade of metabolic events at the level of the respiratory chain of the mitochondria through photochemical events (probably the photoactivation of enzymes), I propose that because of the photochemical and photophysical properties of infrared radiation, infrared radiation starts the cascade of metabolic events by photophysical effects on the membranes (probably the Ca⁺⁺ channels), Action spectra are needed to quantitate the relative effectiveness of the different wavelengths of radiation, since this can help to identify the photoreceptors for the photobiological response, and to establish the optimum conditions (i,e. wavelength, dose, and treatment schedule) for a particular therapy,