The co2-absorbent canisters used in four approved rescue breathing apparatus were investigated. A breathing machine was used to simulate human expiration with gas flow rates of 26, 37, and 48 l/min of 3.5% Co2 in air. The exit-gas co2 concentration, the pressure drop across the canister, and the exit-gas temperatures were monitored. The absorbent capacities of the packed-bed canister designs were found to be dependent on flow rate within the range studied, while the capacities of the other designs were less affected. The radial canister had the least resistance to breathing while the complex bed design proved to be the most effective in utilization of the chemical. The overall results indicated that lioh (lithium hydroxide) has the highest capacity for co2 absorption among the absorbents tested. It can be used in a simple packed-bed design while maintaining acceptable levels of resistance and exit- gas temperature. Examination of oxygen supply with respect to co2- absorption capacity suggested that it is desirable to closely match co2-absorption capacity with the available o2 supply to minimize the size and weight of rescue breathing apparatus.