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A short-term, waterborne exposure and depuration study of Mya arenaria to [ 3 H]tetrachlorodibenzo-p-dioxin([ 3 H]TCDD) was performed. Gill, digestive gland, foot, and gonad were sampled up to 2 weeks after a 24-h exposure to a low dose (10 parts per trillion [pptr]) or high dose (2,000 pptr) of [ 3 H]TCDD in the water. In the gill, peak concentrations occurred at the beginning of depuration (406 and 8,658 pg/g wet weight, low- and high-dose groups, respectively), whereas peak concentrations in the digestive gland and foot were found 12 to 24 h after exposure (digestive gland, 539 and 9,369 pg/g wet weight; foot, 146 and 2,718 pg/g wet weight; low- and high-dose groups, respectively). In contrast, tissue concentrations in the gonad increased through the post-exposure period; at 2 weeks after exposure, tissue concentrations were highest in the gonad (243 and 3,012 pg/g wet weight, low-and high-dose groups, respectively). Examination of gill and gonad by differential display polymerase chain reaction identified potential alterations in expression of genes that may be associated with increased cell cycling or translation initiation. This study shows that M. arenaria can accumulate TCDD in the gonad from an acute exposure, and it identifies several M. arenaria genes whose expression may be altered by TCDD exposure.