Subarachnoid hemorrhage (SAH) was induced by multiple injections of autologous blood into the prepontine cistern in the rabbit. Long-lasting angiographic narrowing was recorded over a period of nine days after SAH. Papaverine (PPV) reversed angiographic narrowing in the first three days after SAH. Vasospasm was refractory to PPV from day five to day nine after SAH. PPV - refractoriness (in vivo) was positively correlated with decreased vessel wall distensibility (in vitro). Arterial segments showed spontaneous increases in tone in the first two days after SAH. Other alterations observed include a marked gradual reduction in the capacity of the vessel wall to contract, reduction in constrictor nerve influences on vascular tone, and impaired acetylcholine - induced vasorelaxation. Tonic contraction to the maximum dose of serotonin was increased in acute spasm and decreased in chronic spasm. It is suggested that the initial cause of arterial narrowing after SAH is the action of vasoactive substances released in the close vicinity of the arterial wall; this then leads to abnormal tone, tissue damage, and structural changes.