The finely lobulated kidneys of American river otters (Lutra canadensis) are not visualized on plain abdominal radiographs. Similar values for blood urea nitrogen (BUN), creatinine, and uric acid were obtained on different analytical systems used in 1984 and 1985. The mean ± SD for measured plasma osmolalities (309.80 ± 8.86 mOsmol/kg) of otters in 1985 was significantly (P < 0.01) less than that of calculated serum osmolalities in the same 1985 specimens (321.61 ± 5.64 mOsmol/kg) and in 1984 specimens (322.20 ± 7.16 mOsmol/kg). Urine specific gravities and osmolalities were highly correlated (r = 0.92). On routine urinalysis, protein and bilirubin were frequent chemical findings, and urobilinogen was present in all urine samples. White and red blood cells and epithelial cells were frequent findings on urine microscopic examinations. Proteus mirabilis was cultured from four of four female otters with genitourinary infections. The mean ± SD creatinine values for paired serum and urine samples (n = 13) were serum creatinine (Scr) 0.66 ± 0.09 mg/dl and urine creatinine (Ucr) 186.9 ± 55.6 mg/dl. Corresponding values for serum electrolytes (Se) and urine electrolytes (Ue) yielded mean ± SD calculated renal fractional clearances (FC = Ue/Se × Scr/Ucr) of sodium 9.65 ± 5.81 × 10−4, potassium 4.15 ± 2.01 × 10−2, chloride 10.81 ± 5.33 × 10−4, calcium 4.52 ± 4.46 × 10−3, and phosphate 6.58 ± 3.44 × 10−3.
