In adult cardiomyocytes (CM) both apoptosis and hypertrophy are mediated by the transcription factor AP-1. SMAD proteins that are only activated by TGFb under apoptotic conditions are able to interact with AP-1 and may turn the balance towards apoptosis. To test this hypothesis we generated an adenovirus encoding Smad4 (AdSmad4) and analyzed its impact on apoptosis induction in comparison to TGFb. TGFb enlarged apoptosis from 7,3 ± 0,7 % in controls to 11,3 ± 0,9 % (n=10, p<0,05). Apoptosis is mediated via AP-1 and SMAD since scavenging of these transcription factors with decoy oligonucleotides inhibited apoptosis. Using specific antibodies in EMSAs indicated that the complex under stimulation with TGFb is mainly composed of Smad2/Smad4. Phosphorylation of Smad2 was detectable after 15 minutes. Infection of isolated CM with AdSmad4 (1000 and 100 MOI) for 24 h and 36 h significantly increased SMAD binding activity to 113.79 ± 11.5 % (n=4 p<0.01) 24 hours after infection with 100 MOI and to 120.26 ± 11.92 % (n=4 p<0.05) with 1000 MOI. This is similar to the activation of SMAD found for TGFbstimulation (113.03 ± 13.64 %, n=4, p<0.05). Infection of CM with AdSmad4 did not increase the number of apoptotic cells after 24 h and 36 h of infection. Conclusion: Smad4 activation alone can not stimulate apoptosis. Therefore, in addition to SMAD, activation of AP-1 as it is found under stimulation with TGFb, is necessary for apoptosis induction in CM.