Eisenmenger's syndrome is associated with a high mortality rate. Pregnancy is particularly hazardous, carrying a death rate of at least 30%. Death is usually attributed to anoxaemia due either to a fall in the systemic vascular resistance or a rise in the pulmonary resistance. On this basis a peripheral vasoconstrictor drug has frequently been recommended as an appropriate form of treatment. To assess this recommendation a patient with Eisenmenger's syndrome was administered the vasoconstrictor drug phenylephrine during diagnostic cardiac catheterisation. This resulted in. a fall rather than a rise in the systemic arterial oxygen saturation. The result provides a caution about the advisability of administering a vasoconstrictor drug to an Eisenmenger patient.