The effects of chronic blockade of single kidney tubules on glomerular capillary pressure (GCP) and blood flow were examined in anesthetized rats. Tubule blockade with castor oil for 24 h decreased GCP from 48.0 +/- 3.8 to 39.5 +/- 4.7 mmHg (P less than 0.001). The full decrease in GCP observed at 24 h was apparent by about 12 h. Bilateral ureteral obstruction for 24 h prevented a decrease in GCP. Nonradioactive microspheres (8.4 +/- 0.84 micron diam) were used to assess relative blood flow to blocked and normal nephrons. These microspheres were completely extracted by rat kidneys and had no effect on total renal blood flow. The ratio of microsphere counts in glomeruli of blocked versus normal nephrons averaged 0.61 +/- 0.21, indicating a significantly (P less than 0.001) decreased glomerular blood flow in nephrons obstructed for 24 h. Since both GCP and blood flow were reduced after tubule obstruction, afferent arteriolar constriction is involved. The data suggest a local mechanism activated by prolonged tubule blockade which shifts filtrate formation and blood flow away from blocked nephrons.