Excretion of porphyrins into bile and urine after intravenous injection of delta-ALA was studied in three patients with catheters inserted into the bile duct because of biliary obstruction and in two healthy volunteers. Excretion of porphyrins into bile increased in response to increasing doses of delta-ALA--up to 18 mumol/kg. On the other hand, excretion into urine reached a constant rate at 2 to 4 mumol/kg delta-ALA and did not increase further with larger doses. These findings suggest that transport of porphyrins across the liver cell membrane to the blood must be an important factor in determining the rate of excretion into the urine. After administration of delta-ALA,excretion of coproporphyrin III into urine increased more than that of coproporphyrin I. As a result, the former was the predominant form of urinary coproporphyrin. When radioactive delta-ALA was administered intravenously, far greater radioactivity was recovered in urine as coproporphyrin III than as coproporphyrin I. These results indicate that only a small portion of urinary coproporphyrin I is derived from the liver. A possible source of coproporphyrin I may be erythropoietic tissues, since the amount of urinary coproporphyrin I was considerably decreased in patients with severely suppressed erythropoiesis.