Despite the possible role of T3 and GH in mediating the effects of in ovo leptin on hepatic activity of iodothyronine deiodinases in the chicken, the direct effect of leptin cannot be excluded, because both hepatic and serum concentrations of leptin-like immunoreactive substance are increased in leptin-treated chickens, especially in males (data not shown). Our in vitro study first confirmed the direct effect of leptin on hepatic activity of iodothyronine deiodinases in chicken hepatocytes. In accordance with the in vivo results, leptin caused a marked increase in D1 and a decrease in D3, at both mRNA expression and activity levels, in primary cultured CEHs.