Mammals are able to convert inorg. arsenic to mono-, di-, and trimethylated metabolites. In previous studies we have shown that the trivalent organoarsenic compds. are more toxic than their inorg. counterparts and that the toxicity is assocd. with the cellular uptake of the arsenicals. In the present study, we investigated cyto-/genotoxic effects of the arsenic compds. arsenate [Asi(V)], arsenite [Asi(III)], monomethylarsonic acid [MMA(V)], monomethylarsonous acid [MMA(III)], dimethylarsinic acid [DMA(V)], dimethylarsinous acid [DMA(III)], and trimethylarsine oxide [TMAO(V)] after an extended exposure time (24 h) and compared the uptake capabilities of fibroblasts (CHO-9 cells: Chinese hamster ovary) used for genotoxicity studies, with those of hepatic cells (Hep G2: hepatoma cell-line). To find out whether the arsenic compds. are bound to membranes or if they are present in the cytosol, the amt. of arsenic was measured in whole-cell exts. and in membrane-removed cell exts. by inductively coupled plasma-mass spectrometry (ICP-MS). In addn., we forced the cellular uptake of the arsenic compds. into CHO-9 cells by electroporation and measured the intracellular arsenic concns. before and after this procedure. Our results show that org. and inorg. arsenicals are taken up to a higher degree by fibroblasts compared to hepatoma cells. The arsenic metabolite DMA(III) was the most membrane permeable species in both cell lines and induced strong genotoxic effects in CHO-9 cells after an exposure time of 24 h. The uptake of all other arsenic species was relatively low (<1% by Hep G2 and <4% by CHO cells), but was dose-dependent. Electroporation increased the intracellular arsenic levels as well as the no. of induced MN in CHO-9 cells. With the exception of Asi(III) and DMA(III) in CHO-9 cells, the tested arsenic compds. were not bound to cell membranes, but were present in the cytosol. This may indicate the existence of DMA(III)-specific exporter proteins as are known for Asi(III). Our results indicate that the uptake capabilities of arsenic compds. are highly dependent upon the cell type. It may be hypothesized that the arsenic-induced genotoxic effects obsd. in fibroblasts are due to the high uptake of arsenicals into this cell type. This may explain the high susceptibility of skin fibroblasts to arsenic exposure.