The mRNA expression pattern of tet family of methyl cytosine dioxygenase enzymes (ten-eleven translocation, tet1, tet2, and tet3), which catalyze the conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) and initiate 5mC remodeling through active demethylation, was investigated in brain, ovary, and liver of Japanese rice fish (Oryzias latipes) adults, and in whole embryos after fertilization to hatching. Moreover, fertilized eggs were exposed to ethanol or 5-azacytidine (5-azaC), 0-2 day post fertilization (dpf), and tet mRNAs on 2 and 6 dpf were analyzed. Our data indicated that all three tet mRNAs were expressed in brain, liver, and ovary of adult fish with substantial variations. In embryos, all three tet mRNAs showed rhythmic expression, high copies in 1-3 dpf followed by down regulation until hatching. Embryonic exposure either to ethanol or 5-azaC was unable to alter the expression of any of these tet mRNAs in 2 dpf; in 6 dpf, there was an increase in tet expression in response to high levels of ethanol (400-500 mM) and 5-azaC (2 mM). Although we have previously documented that both ethanol and 5-azaC was able to induce fetal alcohol spectrum disorder (FASD)-like phenotypic features in Japanese rice fish, the present study indicates that the DNA demethylation by tet may differ between these two compounds.