The reliability of [3H]inulin as a reference substance for the measurement of glomerular filtration rate (GFR) in river lampreys was investigated.

Simultaneously measured renal clearances of [3H]inulin and [14C]-polyethylene glycol (PEG) were not significantly different.

Recoveries of [3H]inulin in the urine following its injection into the proximal tubule and urinary duct averaged 83.5±4.0% (n = 14) and 93.0±4.6% (n = 9) respectively.

No evidence was obtained to suggest penetration of the tubular wall by [3H]inulin following its introduction into peritubular capillaries.

Gel-filtration of lamprey plasma and urine produced no signs of any significant degradation of the inulin to fructose.

[3H]inulin recoveries, though significantly below 100% (P < 0.001), were considered adequate to justify its use after allowing for possible sources of error.

A mean single nephron filtration rate (SNGFR) value of 7.02±0.27 nl/min (n = 89) was obtained from the kidneys of anaesthetized freshwater lampreys.

[51Cr]EDTA was found to be totally unsuitable for the measurement of GFR in this species.

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