Body weight increase together with growth hormone and insulin binding to isolated hepatocytes was used to study the effect of a chronic treatment of dwarf mice with triiodothyronine (T3), bovine growth hormone (GH) and T3 + bovine GH. After 4 weeks of treatment, the body weight increase was similar in all treated groups. However, the T3 and T3 + bovine GH treatment causes a more rapid increase in weight than bovine GH, from the first week of treatment. The binding of growth hormone was four times higher in T3 and T3 + bovine GH than in bovine GH and saline-injected mouse liver cells. This increase was mainly accounted for by a higher number of receptor sites in these cells. At low hormone concentrations, the binding of insulin expressed per unit of cell surface area was 1.3-fold lower in liver cells from bovine GH and T3 + bovine GH than in T3 and saline-injected mice. This decrease was mainly accounted for by a lower number of insulin receptors in these cells. The results presented indicate that bovine GH and T3 treatment affect the body weight increase in dwarf mice according to two mechanisms: (1) T3 increases the number of GH receptors in liver and probably GH content and secretion in pituitary; (2) GH increases the synthesis of somatomedins in liver. In addition, bovine GH treatment decreases the number of insulin receptors.