Videomicroscopic and micropuncture techniques were utilized to determine segmental microvascular responses of in vitro blood-perfused juxtamedullary nephrons to step changes in renal arterial perfusion pressure (PP). At a PP of 104 ± 2 mmHg, inside diameters of arcuate arteries (ARC), interlobular arteries (ILA), and afferent arterioles (AFF) averaged 68.6 ± 6.4, 35.7 ± 1.5, and 20.4 ± 2.3 μm, respectively. Variations in PP within the range of 70-180 mmHg elicited alterations in microvessel diameters with the following slopes: ARC, -0.15 μm/mmHg; ILA, -0.13 μm/mmHg; and AFF, -0.14 μm/mmHg. In other experiments, intravascular pressures were measured during changes in PP. Glomerular capillary pressure was well regulated (slope = 0.19 ± 0.03 mmHg/mmHg), and mid-AFF pressure was partially regulated (slope = 0.60 ± 0.17 mmHg/mmHg); however, pressure measured at the ILA-AFF branch point responded passively to changes in PP (slope = 0.95 ± 0.06 mmHg/mmHg). These observations reveal that, although the entire preglomerular vasculature of juxtamedullary nephrons is capable of actively responding to changes in PP, afferent arterioles are responsible for the predominant resistance adjustment throughout the normal autoregulatory range.