1. Angiographic techniques have been used to study the influence of intracisternally injected haemoglobin on the diameters of the main intrathecal and representative extrathecal (ascending pharyngeal and facial) cranial arteries of the anaesthetized pig. 2. Intracisternal injection of haemoglobin caused concentration-dependent decreases in the diameters of intra-but not extrathecal arteries suggesting that haemoglobin possesses local vasoconstrictor activity. 3. When infused into one ascending pharyngeal artery, acetylcholine (ACh) caused slight dilatation of the intrathecal arteries but no change in the diameters of the ascending pharyngeal and facial arteries. The dilator response induced by ACh in the intrathecal arteries was converted into frank constriction after intracisternal injection of haemoglobin (cerebrospinal fluid concentration approximately 2 x 10 (-5) M). 4. These findings are consistent with the hypothesis that subarachnoid haemoglobin can induce cerebral artery constriction by acting as an extraluminal'sink'for intimally released endothelium-derived relaxing factor (EDRF) and may be relevant to the pathogenesis of vasospasm after subarachnoid haemorrhage in man.