['We tested the hypothesis that oxytocin (OT) contracts blood vessels via vasopressin V1A (VP) receptors, and that this depends on pregnancy.', 'Concentration-contraction relationships (CCR) to OT and VP (10(-12)-10(-6) mol/L) were obtained in different blood vessels. CCR were obtained in uterine arteries (UA) from nonpregnant, mid-pregnant, and late pregnant rats (n = 6-10 per group) in the absence and presence of selective antagonists (10(-7) mol/L).', 'Sensitivity to OT, but not to VP, is attenuated in pregnant rat UA. Antagonists shifted CCR of OT and VP to the right, and, to a lesser extent, of the counterpart, in all UA. VP antagonist depresses oxytocin CCR much more than OT antagonist in pregnant rat UA.'] Does adaptation to pregnancy lead to attenuated rat uterine artery smooth muscle sensitivity to oxytocin?