A simplified photochemical model designed to capture the basic properties of the tropospheric Ox, HOx, NOx, CO, CH4 system is used to study the transition between low and high NOx conditions. These regimes are characterized by substantially different chemical properties and by the existence of a single, stable steady state for each parameter set consistent with low or high NOx conditions. In the parameter space of the system, low and high NOx conditions are separated by a transition zone which corresponds to multiple and/or unstable solutions in the state space of th system. This paper discusses a number of different transitions between low and high NOx conditions which occur as NO and CO source strengths are varied. Emphasis ison the consequence of the existence of a region in parameter space which corresponds to unstable steady states and stable limit cycles in state space. These limit cycles are oscillatory solutions with periods ranging from hundreds to thousands of years. They consist of low and high NOx phases of relatively slowly varying composition separated by transitions in which rapid changes in composition occur. The characteristics of these phases are described in terms of an ozone production cycle based on the chemical reactions assumed to take place. Depending on the phase of the limit cycle, ozone may accelerate or inhibit its own production. The former case represents an autocatalytic cycle with possible ''runaway'' growth of oxidants. The transition between low and high NOx conditions is described with the aid of an ''autocatalytic ratio'' defined in the text.