The unreliability of mammalian glomerular markers in teleostean renal studies.

1. The assumption that (3H) methoxy inulin, (14C) polyethylene glycol (PEG) and (125) iothalamate (glofil) are reliable volume and glomerular markers in teleosts was tested. 2. PEG occupied smaller volumes than inulin and glofil in sea-water-adapted Salmo gairdneri. Ureteral clearances of PEG were about 22% higher than those of inulin and glofil, and urine-to-plasma ratios were significantly greater for PEG than for the other two markers. 3. After introduction into the urinary bladder the three macro-molecules appeared in the plasma, PEG at the lowest rates. 4. These observations indicate that mammalian glomerular markers can penetrate the bladder and possibly the ureters and renal tubules. Therefore, their clearances may not give a true measure of glomerular filtration rates in teleosts.