We administered the diuretics furosemide and ethacrynic acid to conscious freshwater turtles to assess changes in renal function and plasma renin activity (PRA) in an animal which lacks a loop of Henle. Furosemide (2 and 5 mg/kg) produced no changes in blood pressure, hematocrit, plasma electrolytes, glomerular filtration rate (GFR), or PRA. Furosemide doubled urine volume while sodium excretion increased 20-fold and chloride and potassium excretion increased 12-fold (P less than 0.05 in each case). Net potassium secretion was observed. Ethacrynic acid (2 and 5 mg/kg) also produced no changes in blood pressure, hematocrit, plasma electrolytes, or PRA. At the lower dose GFR increased by 40% and urine volume nearly doubled (P Less than 0.05 in each case). Sodium, chloride, and potassium excretion increased roughly 10-fold (P less than 0.05 in each case). At the higher dose, GFR increased by 80% and urine volume more than doubled (P Less than 0.05 in each case). Sodium excretion rose 40-fold, chloride excretion rose 25-fold, and potassium excretion rose 10-fold (P less than 0.05 in each case). At both doses net potassium secretion occurred. The results demonstrate that both drugs inhibit tubular reabsorption in the turtle, acting primarily on distal segments of the nephron. The failure of either drug to alter PRA suggests that the turtle lacks a tubular mechanism for altering renin release.