This paper presents experimental data that show that the biological action of low-intensity laser radiation (LILR) with a wavelength of 808nm (power density P=2.9mW/cm<sup>2</sup>) shows a pronounced dependence on the irradiation regime (continuous or modulated) and the modulation frequency in the range F=1-50Hz. It is shown that sturgeon embryos are convenient objects for estimating the biological activity of the radiation, while the presence of a photobiological effect can be confirmed by testing the size and weight of standard fry and their stability against the action of extreme temperatures (thermal resistance). The dose dependence of the stimulating effect for each irradiation regime is characterized by a curve with a pronounced extremum, whose magnitude and energy position are determined by the frequency modulation. The maximum stimulating action, which manifests itself in a twofold increase of the body mass of the individuals and of their thermal resistance by comparison with the control group, is observed at F=50Hz (dose E=0.17J/cm<sup>2</sup>). The prospects are discussed of using intensity-modulated radiation in medical technologies based on the action of LILR.
© 2008 Optical Society of America
V. Yu. Plavskiĭ and N. V. Barulin, "How the biological activity of low-intensity laser radiation depends on its modulation frequency," J. Opt. Technol. 75, 546-552 (2008)