Cardiac hypertrophy is characterized by an increase in cell size in the absence of cell division and is accompanied by a number of qualitative and quantitative changes in gene expression. Most forms of hypertrophy in vivo are compensatory or adaptative responses to increased workload resulting from various physiological and/or pathological etiologies. Until severe pathological alterations become apparent, myocytes show no drastic morphological changes. On the level of gene expression, upregulation of the so-called fetal genes, i.e., β-myosin heavy chain, α-skeletal and α-smooth muscle actin, and atrial natriuretic factor (ANF) may be observed concomitant with a downregulation of α-myosin heavy chain and the Ca pump of sarcoplasmic reticulum. The use of primary cell culture systems for cardiomyocytes as an in vitro model for the hypertrophic reaction has identified a number of different stimuli as mediators of cardiac myocyte hypertrophy. The molecular dissection of the different intracellular signaling pathways involved herein has uncovered a number of branching points to cytosolic and nuclear targets and has identified many interactions between these pathways. The individual administration of these hypertrophic stimuli, i.e., hormones, cytokines, growth factors, vasoactive peptides, and catecholamines, to cultured cardiomyocytes, reveals that each stimulus induces a distinct phenotype as characterized by gene expression pattern and cellular morphology. Surprisingly, triiodothyronine (T₃) and basic fibroblast growth factor (bFGF) effect a similar cellular phenotype although they use completely different intracellular pathways. This phenotype is characterized by drastic inhibition of myofibrillar growth and by upregulation of α-smooth muscle actin. On the other hand, insulin-like growth factor (IGF) I, a factor promoting muscle cell differentiation, and bFGF, an inhibitor of differentiation, cause completely different cardiomyocyte phenotypes although both are known to signal via receptor tyrosine kinases and have been shown to activate the Ras-Raf-MEK-MAP kinase pathway. However, both IGF-I and bFGF depend on T₃ to bring about their typical responses, i.e., T₃ is permissive for the action of these two growth factors on the expression of α-smooth muscle actin and cell morphology. Most of the hypertrophic stimuli are balanced under normal circumstances in vivo. When this balance is disturbed, however, a pathological heart phenotype may become dominant. Thus the knowledge of signaling pathways and cellular responses triggered by hypertrophic stimuli may be essential for the implementation of therapeutic strategies in the treatment of cardiac hypertrophy.