Many veterinary practices possess an anesthetic machine with a rebreathing system, and therefore the facility to induce anesthesia under more cost-effective reduced fresh gas flow conditions in a semi-closed system. However, as the fresh gas flow is frequently far too high, the rebreathing element is used rarely or not at all, making the anesthesia unnecessarily expensive. The relationships between the fresh gas setting and the final concentrations of expired air are discussed, and experience in 53 dogs with minimal flow anesthesia (500 ml/min), an extreme variant of anesthesia induction using a semi-closed system with minimal excess gas volume and a high proportion of rebreathed gas, is described.