Nephron filtration rate (SNGFR), measured proximally by micropuncture using the technique of total collection of tubular fluid, may be spuriously high as the fall in intratubular pressure during collection may increase the net filtration pressure. Alternatively, the interrupted distal delivery of tubular fluid during proximal collection may up-regulate SNGFR by activating the tubulo-glomerular feedback. The accuracy of nephron filtration measurements was tested in 65 rats, by sampling from the distal and then from the proximal tubule of the same nephrons. Collections were also made in the presence of high concentrations of furosemide at the dose of 10 mg/kg i.v. In 290 nephrons, distal SNGFR was not significantly different from the paired measurements obtained from the proximal tubule (39+/-1 vs 39+/-1 nl/min, p > 0.83). During furosemide, the paired data were 42+/-2 vs 41+/-2 nl/min, p > 0.82, n=92. Different hydration conditions did not influence the results. The correlation between distally and proximally measured filtration rates held for the whole range of measurements (R=0.56; p < 0.0001). Under the present experimental conditions, SNGFR is not spuriously high when measured by total blockade of the proximal tubule. The accuracy of measurements and their reproducibility is not influenced by the site of sampling.