Using a rat model, we investigated the effects of circulating factors in pregnancy on cerebrovascular and systemic vascular function by comparing myogenic reactivity, tone, and endothelial vasodilator production of the posterior cerebral artery (PCA) and mesenteric artery (MA) of nonpregnant (NP) animals perfused with nonpregnant and pregnant human plasma. Arteries from late pregnant (LP) animals were then perfused similarly to evaluate a potential adaptive effect of pregnancy on vessel function. A 3-hour exposure to pregnant plasma caused increased myogenic reactivity and tone in vessels from NP animals and produced a decreased endothelium-derived hyperpolarizing factor response in NP PCAs, findings that were not seen with MAs. The increased reactivity and tone noted in NP vessels was abolished when pregnant plasma was perfused through LP arteries, suggesting these vessels adapt during pregnancy to the vasoconstricting influence of pregnant plasma.