To test the possibility that the ovarian polypeptide hormone relaxin mediates the vasodepressor effect of pregnancy, we measured changes in blood pressure in three groups of spontaneously hypertensive and Wistar-Kyoto normotensive rats: (1) pregnant, bilaterally oophorectomized rats on postmating day 13, (2) pregnant, sham-oophorectomized rats on postmating day 13, and (3) nonpregnant, sham-oophorectomized rats after 13 days. The day before parturition we also measured the pressor responsiveness to angiotensin II and norepinephrine. Systolic blood pressure fell significantly during the last week of gestation in both strains, reaching normotensive levels in the hypertensive rats by term. The pressor responsiveness to angiotensin II, but not norepinephrine, of the pregnant rats was reduced compared with that of the nonpregnant rats in both strains. Oophorectomy did not prevent the fall in blood pressure or the decrease in vascular reactivity to angiotensin II. Therefore, although ovarian relaxin secretion increases during pregnancy, it apparently does not decrease vascular sensitivity to angiotensin II or cause vasodilation.