1. The method of Watson for the estimation of fecal urobilinogen has been modified for use in guinea pigs.
2. Estimations on three or four day fecal collections give less variable results than estimations on single day collections. Loss of urobilinogen consequent upon storage to the third or fourth day is small.
3. For normal guinea pigs a mean daily excretion of 0.23 mg. with a standard deviation of 0.10 was found.
4. Serial estimations were carried out of the urobilinogen excretion of guinea pigs before and during experimental poisoning with lead nitrate. During lead poisoning there developed a statistically significant increased excretion of urobilinogen. This increased excretion was shown, moreover, to be correlated with the fall in hemoglobin levels which occurred in the poisoned animals.
5. It is concluded that these results provide confirmatory evidence for the view that hemolysis plays an important part in the anemia of chronic lead poisoning.