Foods and drinking water are the main sources of human exposure to inorganic arsenic [As(III) and As(V)]. After oral ingestion, the intestinal epithelium is the first barrier to absorption of these species. A human intestinal cell line (Caco-2) was used to evaluate cell retention and transport of As(III) (15.6-156.0 microM) and/or As(V) (15.4-170.6 microM). Cell monolayer integrity, cell viability, membrane damage and effects on cell metabolism were evaluated. Only the highest concentrations assayed [As(III): 156.0 microM; As(V): 170.6 microM] produced a cytotoxic effect with different cellular targets: As(III) altered the permeability of tight junctions, and As(V) caused uncoupling of the respiratory chain. Retention and transport of As(III) was more efficient than that of As(V). After 4h of exposure to As(III) or As(V), monolayer retention percentages varied between 0.87-2.28% and 0.14-0.39%, respectively. Transepithelial transport was greater for As(III) (5.82-7.71%) than for As(V) (not detectable-1.55%). The addition of As(III) and As(V) jointly produced a transport rate similar to that observed when they were added independently.