A study was undertaken to assess whether the consumption of fish and shellfish containing a high concentration (> 1500 micrograms/kg) of organoarsenical compounds was associated with the release of a significant amount of inorganic arsenic (Asi) in vivo. Volunteers were given a known quantity of seafood (ray, cod, plaice, mussel) whose content in total arsenic (As), Asi, monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA) was determined. In addition to the total amount of As excreted within 48 hours, that of Asi and its methylated metabolites (MMA, DMA) was also monitored and compared with that expected on the basis of the results of our previous studies on the toxicokinetics of Asi, MMA, and DMA in humans. Ingestion of ray, cod, or plaice does not seem to be associated with a significant release of Asi in vivo. Following consumption of mussels, however, the amount of DMA excreted in urine is significantly higher than that expected on the basis of the amount of Asi and its methylated derivatives already present in the shellfish. It was also noted that among the different marine organisms analyzed (ray, cod, plaice, sole, sea-bream, mussel), the highest proportion of Asi (on the average 3% of the total) was found in mussels. Further metabolic studies are justified to assess the risk of exposure to Asi associated with the consumption of shellfish. In view of the possible release of dimethylarsenic acid from some organoarsenicals, biological monitoring of occupational exposure to Asi by the measurement of its methylated metabolites can provide misleading results if the workers have consumed some marine organisms within 48 hr before urine collection.