The effects of chronic tubule blockade with paraffin on estimated glomerular capillary pressure (GCP) and single nephron glomerular filtration rate (SNGFR) were examined in anesthetized rats using micropuncture techniques. Blockade of individual proximal and distal tubule lumens for 26 hr resulted in identical reductions in GCP (by about 10 mm Hg). Proximal tubules were also blocked for about 6 hr with and without a hole in the tubular wall just upstream to the block; the hole prevented an increase in proximal tubular pressure. GCP was decreased in the block-plus-hole nephrons, suggesting that an increase in pressure in Bowman's space is not the stimulus leading to a fall in GCP. Blockade of tubules for 26 hr reduced SNGFR from 20.4±1.0 to 12.0±0.8 nl· min^-1· 100 g of body wt^-1. In conclusion, chronic tubular blockade results in reductions in GCP and SNGFR unrelated to changes in tubular pressure. We suggest that this tubuloglomerular feedback response is related to decreased fluid delivery to the distal tubule macula densa.