Several studies have shown that growth hormone (GH) administration can increase size and dimensions of rat hearts. The aim of the present study was to evaluate the effect of GH on the growth of myocytes, myocyte nuclei, connective tissue, connective tissue nuclei and capillaries in the adult rat heart with special reference to total number, total length and total and relative volumes. Eight-month-old female rats were injected with either GH or vehicle for 80 days. Unbiased stereological methods were applied on immersion-fixed left ventricles. GH increased the total number of myocyte nuclei by 25% and the total number of non-myocyte nuclei by 46%. The total lengths of the myocytes and capillaries were increased by 24% and 25% respectively. Furthermore, GH increased the weight of the left ventricle by 58% without changing the relative volume fraction of myocytes, connective tissue or capillaries. In conclusion, GH seems to be a potent mediator of myocardial growth in the adult rat.