1. Unbound or diffusible bilirubin in serum and urine was measured in rats with bile-duct ligation or after continuous infusion of unconjugated bilirubin.

2. The glomerular filtration rate was estimated by measuring the endogenous creatinine clearance and the unbound bilirubin in serum and urine was determined by a Sephadex gel filtration method.

3. Unbound bilirubin clearance was significantly lower than creatinine clearance.

4. The estimated rate of bilirubin reabsorption (calculated from the difference between the unbound bilirubin filtered load and the bilirubin excreted in the urine) was directly related to the serum unbound bilirubin concentration and the unbound bilirubin filtered load.

5. The results suggest that the main mechanism of urinary excretion of bilirubin in the rat involves glomerular filtration and tubular reabsorption by diffusion.

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