The Waters (to-and-fro) system is a commonly used anaesthetic system in small animal clinical practice. Although it may be used as a totally closed system, it has been recommended that it be used as a semi-closed system with relatively high fresh gas flow rates. The performance of the Waters system was studied with reference to wastage of neat, unused fresh gas flow. The results were compared with the performance of a modified Waters system. Using a fresh gas flow rate of 2 litres/minute considerable wastage of fresh gas flow and volatile agent was noted using the Waters system; the wastage was less with the modified system. Marked savings of fresh gas flow and volatile agent could result by reducing the fresh gas flow to 0.5 litres/minute. The findings are discussed in the light of current small animal clinical anaesthetic practice.