Continuous respiratory CO2 gas exchange is simulated theoretically by means of a suitably constructed dilution equation of the form (See PDF) where C is instantaneous concentration, Q quantity flow, and V volume flow. When such an equation is constrained in its numerical solutions to yield inflow-outflow equality for CO2 in the steady state, solutions for CO2 as a time series are found to differ according to whether the site of entry of CO2 is entirely by venous blood or is also by inspired CO2. In the case of entry via the venous blood, increasing CO2 is eliminated while keeping mean CO2 constant with increasing fluctuations about this mean. In the case of entry via the lung, increasing CO2 is eliminated by raising the mean and at the same time oscillations diminish. Submitted on August 13, 1959