Iohexol clearance testing was used to determine glomerular filtration rates (GFR) for 12 juvenile Kemp's ridley turtles (Lepidochelys kempii) that stranded because of cold-stunning. Iohexol was injected intravenously at 75 mg/kg, with subsequent plasma collection at 4, 8, and 24 h post-injection for measurement of iohexol iodine concentrations by inductively coupled plasma mass spectrometry (ICP-MS). Initial GFR was determined on median day 4 of hospitalization (range day 2–5) at an average body temperature of 19°C (66°F) (range 18–21°C [64–70°F]). Mean initial GFR was 4.6 mL/kg/h (range 1.9–8.7 mL/kg/h, n = 10). Convalescent GFR was determined on median day 126 of hospitalization (range day 108–232) at an average body temperature of 24°C (75°F) (range 23–25°C [74–77°F]). Mean convalescent GFR was 13.5 mL/kg/h (range 8.5–21.5 mL/kg/h, n = 10). Statistical analysis of paired initial and convalescent data demonstrated a significantly lower initial GFR, body weight, straight carapace length, and body temperature and significantly greater initial plasma concentrations of iohexol iodine, uric acid, and sodium. Initial GFR was negatively correlated with initial plasma uric acid concentrations. Initial GFR could not be calculated for two turtles because of minimal change in plasma iohexol iodine concentrations over time. Convalescent results could not be determined for two turtles due to death. These results provide the first direct evidence of reduced renal function in cold-stunned turtles.