&NA;In vivo experiments in dogs demonstrated angiographically that the subarachnoid injection of blood produced cerebral arterial spasm both immediately after the injection of blood and 2 days later. The sublingual administration of nifedipine reversed both the acute and the delayed cerebral arterial spasm. In addition, sublingual administration of nifedipine 20 minutes before the subarachnoid injection of blood prevented the acute spasm.