1. 1. "Brown pigments" from human and rat excreta (urine, feces, and bile), human and bovine gallstones, and commercial bilirubin have been isolated and characterized by elementary analysis, mass spectra, absorption spectra, potassium permanganate and chromic acid oxidation. The results provide further evidence for the pyrrolic nature of "black pigments." 2. 2. [4-14C]ALA administered to a normal person was incorporated into urochromes A and B within 24 hr; the maximum labeling was found in urochromes 24 hr before maximum labeling of the mesobilifuscin. [4-14C]ALA administered to rats was excreted within 1 hr (maximum 35 min) as 14CO2; the total amount added up to only 2% of administered activity. 3. 3. Urine and feces from female rats treated with N-2-fluorenylacetamide (2-FAA) and N-OH-2-FAA have been collected and absorption changes were measured directly in urine; absorption was assumed to be proportional to urochrome concentration in urine. In urine from rats treated with 2-FAA absorption increased appreciably until tumor was found, then the absorption decreased slightly. In urine of N-OH-2-FAA treated rats absorption decreased slightly, until the tumors were detected and then the decrease was more pronounced.