Relationships between alterations in the profile of urinary arsenic (As) species and the presence of cutaneous signs of arsenicism were studied in Region Lagunera, Mexico. The use of urinary concentrations of putative substrates and products of the As metabolism pathway, as indicators of metabolic efficiency is also discussed. Arsenic was determined by hydride generation atomic absorption spectrophotometry and separation of As species was performed by ion exchange chromatography. The exposed group had an average of 0.408 mg As/l of total As (TAs) in their drinking water, whereas "control' individuals had 0.031 mg/l. Urinary concentrations of arsenic species and TAs were 20 to 95 times higher in the exposed group. Significant increases in the relative proportions of inorganic arsenic (Asi) and monomethylarsonic acid (MMA), accompanied by decreases of dimethylarsinic acid (DMA) were also found in exposed individuals. Therefore, significant decreases in the value of the MMA/Asi, DMA/MMA and DMA/ Asi ratios were observed, suggesting a decreased As methylating ability. Exposed individuals bearing cutaneous signs had a significantly longer time of exposure, higher urinary concentrations and proportions of MMA and MMA/Asi values, and significantly lower DMA/ MMA than exposed individuals without cutaneous signs. Further research is needed to identify better parameters for assessing the efficiency of As metabolism in chronically exposed populations and to confirm the potential relationship between metabolic alterations and overt signs of As toxicity.