1. Lung pressures, buccal pressures, lung volumes, and EMGs from respiratory muscles were measured in unrestrained Xenopus laevis to analyse their roles in the lung ventilation cycle.
2. Lung pressure was always maintained above atmospheric levels and a buccal pumping mechanism was used to fill the lungs in Xenopus, as in other Amphibia.
3. Xenopus, unlike other amphibians, does not ventilate the buccal cavity between lung ventilations.
4. Expiration of gases from the buccal cavity is aided by muscles which decrease buccal cavity volume. Other anurans increase buccal cavity volume during expiration.
5. The buccal phase of inspiration occurs after expired air has passed from the lung and buccal cavity, in comparison to the ranids and bufonids which inspire fresh air into the buccal cavity before expiration.