Specific binding sites for growth hormone (GH) have been found in a variety of tissues. The rabbit liver GH receptor has recently been purified, sequenced and cloned. It is a polypeptide chain of 620 aminoacids with a single transmembrane domain. The sequence of the extracellular hormone binding domain of the receptor is identical to the sequence of the serum binding protein. The intra-cellular signalling mechanism is not known. The GH receptor gene(s) has to be studied. Rapid and great changes in the number of GH receptors have been demonstrated in situations of growth defect in rats. GH and insulin both play an important role in the regulation of GH sites, but the mechanisms of regulation remain to be clarified. Studies with monoclonal antibodies against the GH receptor and also sequence analysis and cloning of cDNA from different tissues should help to answer the question of the multiplicity of the GH receptors.