We previously reported that two splicing variants of human AS3MT mRNA, exon-3 skipping form (Delta 3) and exons-4 and-5 skipping form (Delta 4,5), were detected in HepG2 cells and that both variants lacked arsenic methylation activity (Sumi et al., 2011). Here we studied whether hydrogen peroxide (H2O2) triggers alternative splicing of AS3MT mRNA. The results showed that exposure of HepG2 cells to H2O2 resulted in increased levels of a novel spliced form skipping exon-3 to exon-10 (Delta 3-10) in an H2O2 concentration -dependent manner, although no change was detected in the mRNA levels of Delta 3 AS3MT. We found decreased protein levels of serine/arginine-rich 40 (SRp40), which we determined to be a candidate splice factor for controlling the splicing of AS3MT mRNA. We next compared the amounts of methylated arsenic metabolites between control and H2O2-exposed HepG2 cells after the addition of arsenite as a substance. The results showed lower levels of methylated arsenic metabolites in HepG2 cells exposed to H2O2. These data suggest that the splicing of AS3MT pre-mRNA was disconcerted by oxidative stress and that abnormal alternative splicing of AS3MT mRNA may affect arsenic methylation ability. (C) 2016 Elsevier Inc. All rights reserved.