The influence on pain perception of acute and chronic exposure to infrared laser radiation applied to the skulls of rats is examined. The experiment was carried out on 60 Wistar white male rats. A semiconductive infrared laser (wavelength -- 904 nm, frequency -- 100 Hz, pulse duration -- 10 ns, mean power 10 mW, energy density 1.5 J/cm2) was used. A skull fornix of rats was irradiated with use of a multidiode probe 10 minutes daily for 14 consecutive days. The pain perception was determined by the latency of foot-licking or jumping from the surface of a 56 degree(s)C hot plate. The measurements were made immediately before irradiation, at 1, 5, 10, 15, 20, 25 and 30 minutes after irradiation, and then every 15 minutes until 120 minutes after irradiation. The pain perception is estimated immediately after irradiation, 24 hours after exposure at the 7th and 14th day of irradiation as well as at the 7th and 14th day after the last irradiation. A pretreatment with Naloxone (1 ml/kg of body weight i.p.) was made in a part of the animals in order to evaluate the involvement of the opioid system in the observed effect. Obtained data show that low-energy laser irradiation affects the pain reactivity of rats. The inhibition of the analgesic effect of laser irradiation by the antagonist of opioids -- Naloxone -- and also the prolonged character of this action as a result of direct irradiation of the skull indicate the involvement of the opioid system in the occurrence of laser-induced analgesia.