Previously we have shown that growth hormone (GH) increases the total number of myocyte nuclei of the left ventricle in adult rats (8 months old). In the present study, we investigated whether GH could increase the total number of myocyte nuclei of the left ventricle in young and old rats. Female rats, 3 months old and 20 months old, were injected with GH or vehicle for 80 days. Using immersion-fixed left ventricles, unbiased stereological methods were applied. The weight of the left ventricle was increased by 49% (P<0.001) in the GH-injected young rats and by 32% (P<0.01) in the GH-injected old rats compared with the controls. Compared with the control groups, there was a 31% increase in the total number of myocyte nuclei in the GH-injected young group (P<0.05), but no significant increase in the GH-injected old group. The total number of non-myocyte nuclei was increased by 59% in the young GH-injected group (P<0.001) and by 25% in the old GH-injected group (P<0.01). In conclusion, GH induced a substantial left ventricle growth in both young and old rats. GH increased the total number of myocyte nuclei in the left ventricle of young rats, but not in old rats. This study shows that the myocyte response to GH declines with ageing.