The effect of i.v. ketamine on arterial pressure and heart rate were examined in pithed rats, rabbits and cats. In all three species ketamine caused a brief decrease in arterial pressure and heart rate. In the rat, but not in the other two species, this initial decrease in arterial pressure was followed by a pressor response which was resistant to alpha-adrenoceptor blockade, depletion of tissue noradrenaline stores and adrenalectomy. It is concluded that the peripherally mediated pressor response found in the pithed rat is specific to this species, is not a result of liberation of peripheral catecholamines and does not explain the pressor effect of ketamine found in man and several animal species.