[Spatiotemporal organization of c-fos expression in the brain of rats during the early stages of learning]. The expression of c-fos mRNA in the brain of rats during the formation of conditioned reflex to weak electric stimulation was studied. It was found that expression of c-fos mRNA in all the regions of the brain was maximal during the stages of habituation and stabilization of the positive reinforcement during a 10-min interval and was reduced under the conditions of a long-term positive reinforcement. A reduction in the number of c-fos-positive cells was observed with a delay in time, with the distance from the conditioned and unconditioned stimuli, and with an increase in the conditioned reaction. The data obtained indicate the possible existence of specific mechanisms of organization of processes of the activity of certain centers of the brain, taking part in formation of conditioned reflexes. The data obtained testify to the use of c-fos as a marker of activation of the brain structures during the formation of conditioned reflexes.