Cerebrovascular reactivity to biogenic amines varies in relation to both arterial diameter and age. The present study examines the hypothesis that these patterns of reactivity are secondary to corresponding variations in the Ca2+ sensitivity of the contractile proteins. To test this hypothesis, we permeabilized segments of common carotid (Com), basilar, main branch middle cerebral, and second-branch middle cerebral (MCA-B) arteries from nonpregnant adult and near-term fetal sheep using beta-escin. Permeabilization methods were carefully validated and adjusted for each artery type. Baseline myofilament Ca2+ sensitivity in both adults and fetuses increased significantly from the Com to the MCA-B and was generally higher in fetuses than in adults. Serotonin dose dependently increased Ca2+ sensitivity via a G protein-dependent mechanism in all arteries. The magnitudes of this effect did not vary among artery types but were significantly greater in fetal than in adult arteries. This effect of serotonin was mimicked by guanosine 5'-O-(3-thiotriphosphate), a nonhydrolyzable analog of guanosine 5'-triphosphate, and its effects were also much greater in fetal than in adult arteries. We conclude that patterns of cerebrovascular reactivity to biogenic amines were determined, at least in part, by underlying variations in baseline myofilament Ca2+ sensitivity and/or its alteration by G protein-dependent mechanisms.