DESCRIPTION: Baroreceptor reflex functioning is under the influence of several factors including steroid hormones. In pregnancy, there is an attenuation of the baroreceptor reflex such that in late pregnancy, animals are unable to maintain pressure during hemorrhage. Since hemorrhage is a common occurrence in labor and delivery, the importance of understanding this observation is noteworthy. The hypothesis to be tested in the current proposal is that this deficit occurs in the brain or efferent part of the reflex pathway whereas the afferent pathway is not altered. Conscious rabbits will be studied to determine if sympathetic activity is less in pregnant animals during hemorrhage. Assessments of sympathetic activity include, renal nerve recordings and regional resistances, plasma catecholamines, total and renal norepinephrine spillover, and adrenal and ganglionic levels of tyrosine hydroxylase mRNA. Responsiveness of single arterial baroreceptor fibers and efferent responses to stimulation of the aortic depressor nerve will be determined. Vasodilator responses of isolated resistance vessels will also be assessed. The timing of the above changes during pregnancy and lactation will be studied.