Arsenic methylating enzyme (AS3MT) genetic variations and their role in variability of As metabolism were evaluated in population of pregnant/non-pregnant women exposed to low levels of As. We have tested the associations between genotypes/haplotype of seven AS3MT SNPs and As metabolites in urine (methylation efficiency). Significant associations of individual SNPs and protective haplotype with arsenic metabolites have been confirmed among non-pregnant women. On the contrary, these associations have been almost absent among pregnant women – most possibly a counsequence of physiological changes in pregnancy, and/or impact of external DMA due to higher seafood intake. Both factors can influence As metabolism.