Both α‐adrenoceptor‐ and β‐adrenoceptor‐stimulation lead to hypertrophic growth of the myocardium. But only β‐adrenoceptor‐stimulation requires the pre‐cultivation of cells with active TGF‐α. In order to define signalling molecules that are specifically involved in α‐adrenoceptor‐dependent hypertrophy, changes in expression and hypertrophic responsiveness during pre‐cultivation with TGF‐α were investigated. Isolated adult ventricular cardiomyocytes from rats were either cultured in 20% (v/v) foetal calf serum (FCS) to activate autocrine released TGF‐β or used without pre‐treatment. Protein synthesis was analysed by ¹⁴C‐phenylalanine incorporation. Expression of signalling molecules was determined by immunoblotting. During cultivation of cardiomyocytes with active TGF‐β only the expression of p38 MAP‐kinase increased. Subsequent stimulation of β‐adrenoceptors induced protein synthesis in a p38 MAP‐kinase‐dependent way. However, stimulation of β‐adrenoceptors activated p38 MAP‐kinase irrespective of pre‐treatment with TGF‐β. In the absence of this cytokine, hyperosmolarity or reconstitution of mechanical activity increased protein synthesis via p38 MAP‐kinase activation in freshly isolated cells. In conclusion, activation of p38 MAP‐kinase is a newly identified necessary signalling step required for β‐adrenoceptor induced hypertrophic growth. Like activation of adenyl cyclase, activation of p38 MAP‐kinase is up‐stream of the TGF‐β‐induced coupling to the regulation of protein synthesis. Reconstitution of mechanical activity mimics the co‐activation required and induced by TGF‐β.