The intravenous administration of the antiadrenaline drug phenoxybenzamine (Dibenzyline) markedly raised the arterial adrenaline and noradrenaline concentration in dogs lightly anaesthetized with thiopentone. Graded haemorrhage led to a further rise in the amounts of amine. In adrenalectomized dogs, phenoxybenzamine moderately increased the plasma noradrenaline concentration. During haemorrhagic hypotension, previous treatment of adrenalectomized animals with phenoxybenzamine led to a significantly greater rise in plasma noradrenaline compared with that of adrenalectomized animals subjected to haemorrhage without treatment with phenoxybenzamine. Thus, phenoxybenzamine (1) raised plasma amine concentration largely due to adrenal medullary stimulation, and (2) led to increased plasma noradrenaline concentrations during sympathetic stimulation in adrenalectomized animals. The previous administration of phenoxybenzamine reduced the amount of blood which could be withdrawn before final circulatory collapse in both normal and adrenalectomized dogs.