ABSTRACT: The present study investigated arsenicâs effects on mummichogs (Fundulus heteroclitus), while also examining what role that gender or age of exposure might play. Adult male and female mummichogs were exposed to 172ppb, 575ppb, or 1,720ppb arsenic as sodium arsenite for 10 days immediately prior to spawning. No differences were noted in the number or viability of eggs between the groups, but there was a significant increase in deformities in the 1,720ppb arsenic exposure group. Total RNA from adult livers or 6-week-old juveniles was used to probe custom macroarrays for changes in gene expression. In females, 3% of the genes were commonly differentially expressed in the 172 and 575ppb exposure groups. In the males, between 1.1-3% of the differentially expressed genes were in common between the exposure groups. Several genes, including apolipoprotein, serum amyloid precursor, lysozyme, and tributyltin-binding protein, were commonly expressed in either a dose-responsive or dose-specific, but across genders, manner. These patterns of regulation were confirmed by QPCR. These findings will provide us with a better understanding of the effects of dose, gender, and exposure age on the response to arsenic. Adult mummichogs (Fundulus heteroclitus) were exposed to either 0 or 172ppb sodium arsenite for 10 days before the full moon. Each treatment group had 5 20-gallon plastic tanks with 2 male and 5 female fish per tank. The fish were exposed via a static renewal, with renewals every 48 hours. Mummichogs were housed at 25C in a 14/10 light/dark cycle at 18% salinity. On the 3 days surrounding the full moon, spawning substrates were provided and eggs collected. Adult males and females: After the last day of egg collection, the adults were euthanized. The livers were removed from the males and females, placed in TRI-Reagent® for RNA isolation (Sigma Chemical Co, St. Louis, MO), and then stored at -80C. Juveniles: Eggs were transferred to clean water and allowed to hatch. The juveniles were grown out in clean water for 6 weeks and then were euthanized. Whole juveniles (7-10 per exposure per tank) were placed in TRI-Reagent® for RNA isolation (Sigma Chemical Co, St. Louis, MO), and then stored at -80C.