Abstract. This paper contributes to the study of Freely Rewriting Restarting Automata (FRR-automata) and Parallel Communicating Grammar Systems (PCGS) as formalizations of the linguistic method of analysis by reduction. For PCGS we study two complexity measures called generation complexity and distribution complexity, and we prove that a PCGS Π, for which both these complexity measures are bounded by constants, can be simulated by a freely rewriting restarting automaton of a very restricted form. From this characterization it follows that the language L(Π) is semi-linear, that its characteristic analysis is of polynomial size, and that this analysis can be computed in polynomial time.