This study compared the vasodilatory response to magnesium sulfate (MgSO₄) of cerebral and mesenteric resistance arteries and determined if the response varied between different gestational groups. Third-order branches (<200µm) of the posterior cerebral (PCA) and mesenteric arteries (MA) were dissected from non-pregnant (NP, n=6), late pregnant (LP, day 19, n=6), and postpartum (PP, d3, n=6) Sprague-Dawley rats. A concentration-response curve was performed by replacing the low MgSO₄ (1.2 mM) HEPES buffer solution with increasing concentrations of MgSO₄ (4, 6, 8, 16, 32 mM) and measuring lumen diameter at each concentration. All groups exhibited concentration-dependent dilation to MgSO₄, decreasing the amount of tone in the vessels. However, MA were significantly more sensitive to MgSO₄ than PCA. While there was no difference in response between different gestational groups in MA, the PCA from the LP and PP groups showed a significantly diminished response to MgSO₄. The percent dilation at 32mM MgSO₄ for PCA vs. MA in NP, LP and PP animals was: 36±2 vs.51±7% (p<0.05); 19±9 vs. 54±6% (p<0.01 vs. PCA and NP) and 12±5 vs. 52±11% (p<0.01 vs. PCA and NP). These results demonstrate that MgSO₄ is a vasodilator of small resistance arteries in the cerebral and mesenteric vascular beds. The refractory responses of the PCA in LP and PP groups demonstrate changes in the cerebrovascular vasodilatory mechanisms with gestation. The greater sensitivity of the MA to MgSO₄-induced vasodilation suggests that the prophylactic effect of MgSO₄ on eclamptic seizures may be more closely related to the lowering of systemic blood pressure than to an effect on cerebral blood flow.