Whether vasospasm results from smooth muscle contraction or from arterial wall infiltration by cells and other material is subject to debate. Computer-assisted image analysis was used to measure lumen area, total wall area, and area of tunica media plus tunica intima of cross-sections of monkey right middle cerebral arteries (MCAs), exposed in vivo for 6 days to whole blood (n = 4), oxyhemoglobin (OxyHb, n = 5), methemoglobin (MetHb, n = 5), bilirubin (n = 5), mock cerebrospinal fluid (CSF, n = 6), and supernatant fluid from an incubated mixture of autologous blood and mock CSF (n = 5). Five control (left) MCAs from each group and 4 MCAs contracted in vitro with potassium chloride were measured. Significant angiographic vasospasm occurred in groups receiving whole blood, supernatant fluid, and OxyHb (p < 0.05). There was significant correlation (r = 0.58, p < 0.05) between right MCA diameter on angiography and diameter calculated from lumen area. When compared to effects of mock CSF, OxyHb significantly increased total wall area. When right and left MCAs were compared within groups, total wall area increased in every group with significant increases in groups exposed to mock CSF, OxyHb, and bilirubin (p < 0.05). No changes developed in area of tunica media plus tunica intima, whether comparing right versus left MCAs within groups or right MCAs between groups. Contraction in vitro did not significantly increase total wall area or area of tunica media plus tunica intima. Light