Abstract <p>The paper considers a model describing the evolution of the active medium. The model is a cellular automaton with stochastic rules for the transition of a cell from one state to another depending on its state and the states of other cells of the automaton. The model describes the transport of activity in such an environment. It is shown that for certain values of the parameters that define the transition rules, a phase transition from a state of rest to a state of bursts of activity is observed in the behavior of the system. The behavior of the system in the vicinity of the phase transition was studied and it was shown that it exhibits signs of critical self-organization. Automata with different systems of interrelations between cells were studied: both regular lattices and automata with random connections between cells. It is shown that critical behavior is characteristic of all such systems.</p>