Arsenic toxicity is species-dependent for mammals and is transformed to other metabolites in various organs. In order to understand its metabolism, in-vitro simulation experiments were set up for various organ tissues. As(V) was transformed to As(III), while DMA(V) was changed to DMAS in sheep rumen, but the suspected MA-V) was not formed. This is different for the incubation of seaweed in rumen fluid, which points to a different unknown source for MA(V). In another study, dog MDCK kidney cells showed that inorganic arsenicals were methylated in kidney cells while the exposed DMAS was only oxidized to DMA(V) in kidney cells, and the exposed DMA(V) rarely transformed. The toxicities of arsenicals in dog kidney cells is dependant upon the uptake rates and the transformation of arsenicals. DMAS was as toxic as the inorganic arsenic species and an order to magnitude more toxic than the oxo-species. Methylated arsenic species can accumulate in hair and wool and can be used as a biomarker for arsenic ingestion. When methylated compounds have different arsenic species as precursor material, such as arsenosugars, more DMA(V) and DMAS are found in keratinous tissues. Here, the horn and wool from the seaweed-eating sheep were used as a study object. Sheep horn can be used for monitoring arsenic exposure. The penetration of arsenic through human skin showed that As(III) and DMA(V) penetrate the quickest through skin while only As(III) and As(V) accumulated in skin. As(III) and DMA(V) were 40 times faster at penetration than As(V) and arsenosugars. The incubation of exposure matrix, for example seaweed extract, can protect arsenic penetration when DMA(V) or arsenosugars are being investigated, but it does not work well in As(III) contaminated seaweed extract.