This study aimed to establish the relations between growth hormone and serum somatomedin during therapy with human growth hormone (hGH) of pituitary dwarfs. In untreated subjects the levels of somatomedin were low. After one intramuscular injection of hGH the levels of somatomedin raised after the 6th hour, whereas the hGH concentrations were again near basal values, and continued to raise up to a maximum value at the 24th or 30th hour. During long term therapy, somatomedin increases up to normal, and the correlation between somatomedin levels and growth rate is significant. After two semesters of therapy, the growth rate and the somatomedin levels decrease together, and, in one case at least, seem dependant on the apparition of anti-hGH antibodies. Abbreviations hGH: human growth hormone SM: somatomedin 1 Supported by Institut National de la Santé et de la Recherche Médicale.