Previous studies have demonstrated the critical role of the afferent arteriole in autoregulation of nephron blood flow in response to changes in perfusion pressure. The present study focused on the responses of postglomerular vascular segments to alterations in renal arterial pressure. Afferent arterioles, efferent arterioles and outer medullary descending vasa recta of juxtamedullary nephrons were visualized using the in vitro blood-perfused juxtamedullary nephron technique. Simultaneous measurements of inside vessel diameter and centerline erythrocyte velocity were made in order to determine single vessel blood flow. Blood flow measured in afferent arterioles (N = 13) displayed efficient autoregulation of blood flow and afferent arterioles responded actively with decreases in arteriolar diameter during stepwise elevations of renal perfusion pressure from 100 to 150 mm Hg. Similarly, blood flow measured at efferent arterioles (N = 9) exhibited autoregulation during increases in renal perfusion pressure. However, efferent arteriolar diameters were not altered during increases in perfusion pressure. During superfusion with the calcium channel blocker, diltiazem (10 microM), which primarily dilates afferent arterioles, efferent arteriolar blood flow (N = 7) increased and responded to changes in perfusion pressure. Nevertheless, efferent arteriolar diameter remained unchanged and did not respond to increases in perfusion pressure. Outer medullary descending vasa recta (N = 7) diameter, centerline erythrocyte velocity and calculated blood flow were also not significantly altered following stepwise increases in pressure to 125 and 150 mm Hg. These data demonstrate effective autoregulation of postglomerular blood flow, measured at efferent arterioles and at outer medullary descending vasa recta, over a perfusion pressure range of 100 to 150 mm Hg. There was no dissociation of arteriolar and outer medullary descending vasa recta blood flow responses to increases in renal perfusion pressure indicative of efficient autoregulation in both cortical and medullary postglomerular circulations of the rat.