In a laboratory model, humidity output was investigated in relation to the design of a circle absorber system. A 70-kg subject was simulated with fresh gas flows of 0.5, 2 or 5 litre min-1. Different circle systems, absorption canisters and tubings were studied. It was found that both the type of circle system (according to Eger's classification) and the size of the absorption canister, influenced the humidity of inspired gases. Coaxial tubing only moderately increased the humidity. If a fresh gas flow of 0.5 liter min-1 was used, optimum moisture contents were attained, irrespective of the circle system tested. Low fresh gas flows, a small canister and an Eger A type circle system, were factors which increased humidification.