The aim of the study was to quantify cerebral vasospasm in rats after subarachnoid hemorrhage (SAH) by morphometric examination of basilar artery and to evaluate the influence of Phosphoramidon on basilar artery constriction. The rat cisterna magna (CM) was cannulated and after 7 days SAH was developed by administration of 100 microliters autologic, non-heparinized blood to the CM. The sham subarachnoid hemorrhage was developed by intracisternal administration of 100 microliters of artificial cerebrospinal fluid. After 60 min and after 24 h Phosphoramidon was injected into the CM in a dose of 40 nmol diluted in 50 microliters of cerebrospinal fluid. After perfusion, the brain was removed from the skull and histological preparations of the basilar artery were made. The internal diameter and wall thickness of basilar arteries were measured by interactive morphometric method. The most severe vasospasm was found in rats after SAH and the administration of Phosphoramidon in the late phase after SAH caused the dilatation of the basilar artery. The presence of numerous infiltrations composed of neutrophils and macrophages correlated with advanced vasospasm (index of constriction 5 times lower than normal), suggesting the role of other factors participating in the late phase of vasospasms after SAH.