The effect of topically applied papaverine (PAP) and norepinephrine (NE), on the microvascular pressure distribution was studied in the cremaster muscle of anesthetized rats. The cremaster muscle was exteriorized into a tissue bath containing Krebs bicarbonate buffer, and microvessel diameters and pressures were measured using a video caliper and the resistance servo-null method, respectively. Pressures were measured in the 1st through 4th branch order arterioles (1A to 4A) and 1st through 4th branch order venules (1V to 4V). Resistances were calculated across each segment from pressure gradients and 1A blood flow which was estimated from red cell velocities. PAP (10 microM) produced approximately a 15% decrease in arteriolar pressures with a concomitant increase in venular pressures of between 15 and 50%. In contrast, NE (0.1 microM) significantly increased arteriolar pressure by approximately 30% and decreased venular pressure by about 25%. Changes in systemic pressure during treatment with PAP and NE were small or insignificant and could not account for the observed changes in microvascular pressure. Significant dilation was observed in 3A, 4A, and 3V vessels after papaverine. In comparison, NE caused significant constriction in all vessel orders except 4V. PAP decreased resistance across all segments between 1A and 4V by 22-42% and increased venular resistance by almost 400%. NE increased resistance in all microvascular segments with the largest changes occurring in 2A to 3A (+276%) and 4A to 4V (+277%) segments. These data demonstrate that PAP and NE induce significant and opposite changes in arteriolar and venular pressures. Such network alterations in microvascular pressure should be considered when evaluating microvascular reactivity and exchange in the presence of vasoactive agents.