1. The inactivation of noradrenaline and prostaglandin E2 was studied in the pulmonary circulation of anaesthetized dogs.
2. Under chloralose anaesthesia and ventilation with air, the inactivation of noradrenaline was 20% and that of prostaglandin E2 was 91%. These values are in agreement with results from previous work.
3. When the dogs were ventilated with halothane—air mixtures, the inactivation of prostaglandin E2 was unaffected but that of noradrenaline was significantly reduced.
4. This effect of halothane is probably due to an interaction with the transport system for noradrenaline associated with cell membranes.
5. Analogous changes in pulmonary noradrenaline inactivation could occur in patients anaesthetized with halothane.