Hepatic thyroxine (T4) to tri-iodothyronine (T3) conversion was measured in six animal groups: Group A was fed a severe zinc-deficient diet (1.98 ppm) for 5 weeks; group B was a pair-fed control group for group A; group C was fed a less severe zinc-deficient diet (6.10 ppm) for 5 weeks; group D was a pair-fed control group for group C; group E was fed a zinc-supplemented control diet (90.4 ppm) for 5 weeks; and group F was first fed the severe zinc-deficient diet for 5 weeks and then placed on the zinc-supplemented control diet until a body weight corresponding to the final weight of group E was obtained. Serum T3 and T4 levels and T4 to T3 conversion were significantly reduced in group A. A significant positive correlation was observed between T4 to T3 conversion and alcohol dehydrogenase (zinc-metalloenzyme) activity in liver tissue from the six groups. Thus, it appeared that an appropriate amount of zinc might be one of the factors in thyroxine to tri-iodothyronine conversion in liver tissue normally.