The effects of thyroid hormones on hepatic insulin-like growth factor (IGF) II gene expression and their interaction with cortisol in the ontogenic control of this gene were investigated in fetal sheep during late gestation (term 145 +/- 2 days) and after experimental manipulation of fetal plasma hormone concentrations. In intact fetuses, a significant decrease in hepatic IGF-II mRNA abundance was observed between 127-130 and 142-145 days of gestation, which coincided with the normal prepartum rise in plasma cortisol and triiodothyronine (T3) concentrations. This ontogenic decline in hepatic IGF-II gene expression was abolished in fetuses in which the prepartum rise in plasma T3, but not cortisol, was prevented by fetal thyroidectomy. At 127-130 days, downregulation of hepatic IGF-II mRNA abundance was induced prematurely in intact fetuses by an infusion of cortisol for 5 days (2-3 mg . kg-1 . day-1 iv). Plasma concentrations of cortisol and T3 in the cortisol-infused intact fetuses were increased to values seen close to term. Similar findings were observed in thyroidectomized fetuses, in which, despite thyroidectomy, cortisol infusion significantly increased plasma T3 concentrations and caused a premature decrease in hepatic IGF-II mRNA levels. However, in intact fetuses at 127-130 days, the increasing of T3 concentrations alone by exogenous T3 infusion (8-12 microg . kg-1 . day-1 iv for 5 days) had no effect on hepatic IGF-II mRNA levels. Overall, a decrease in hepatic IGF-II mRNA abundance was only observed in fetuses in which there were concurrent increases in plasma cortisol and T3 concentrations. When observations from all fetuses were considered, irrespective of gestational age or treatment, hepatic IGF-II mRNA levels were negatively correlated with plasma cortisol and T3 but not thyroxine concentrations. Partial correlation analysis of hepatic IGF-II, cortisol, and T3 values showed that the plasma concentration of cortisol in the fetus had the predominant effect on hepatic IGF-II mRNA abundance. These findings show that T3 may mediate, in part, the maturational effects of cortisol on hepatic IGF-II gene expression but that it is ineffective without a concomitant rise in fetal plasma cortisol. Hence, increased concentrations of both cortisol and T3 appear necessary to induce downregulation of hepatic IGF-II mRNA abundance in fetal sheep close to term.