1. A detailed study of the urinary excretion pattern of porphyrins in humans chronically exposed to As via drinking water was performed using high performance liquid chromatography (HPLC) 2. Thirty-six individuals (15 men and 21 women) were selected from a town which had 0.400 mg L-1 of As in drinking water. The control group consisted of thirty-one individuals (13 men and 18 women) whose As concentration in drinking water was 0.020 mg L-1. 3. The major abnormalities in the urinary porphyrin excretion pattern observed in arsenic-exposed individuals were: (a) significant reductions in coproporphyrin III excretion resulting in decreases in the COPRO III/COPRO I ratio, and (b) significant increases in uroporphyrin excretion. Both alterations were responsible for the decrease in the COPRO/URO ratio. 4. No porphyrinogenic response was found in individuals with urinary As concentrations below 1,000 micrograms of As g-1 of creatinine. However, as arsenic concentrations exceeded this value, the excretion of porphyrins (except coproporphyrin III) increased proportionally. 5. The prevalence of clinical signs of arsenicism showed a direct relationship to both As concentration in urine and time-weighted exposure to As. A direct relationship between time-weighted exposure and alterations in urinary porphyrin excretion ratios was also observed. 6. The alterations found are compatible with a lower uroporphyrinogen decarboxylase activity in arsenic-exposed individuals. However, the similarities in the urinary porphyrin excretion pattern between As-exposed individuals and Dubin-Johnson syndrome patients suggest that impairments in the excretion of coproporphyrin isomers may also contribute to the pattern observed.