Growth hormone (GH) increases plasma T3 and decreases plasma T4 in 18-day old chicken embryos, in newly hatched chicks and in adult chickens within 2 h after injection. The in vivo increase in T3 can be linked to an increased in vitro T3 recovery from liver homogenates incubated with T4. Specific type I and type III deiodinase tests (5'D-I and 5D-III), however, show that GH has no effect al all on the amount of hepatic type I enzyme (catalyzing T4 deiodination to T3) but acutely decreases the amount of type III enzyme (catalyzing T3 deiodination). This suggests that the GH-induced increase in plasma T3 is not due to an increased T3 production, but is the result of a decreased T3 breakdown. The lack of a stimulatory effect of GH injection in 3-day-old fed chicks might be the combined result of a low hepatic type III enzyme level and a low GH receptor availability at that stage.