The growth factor TGFb is released after myocardial infarction and contributes to cardiac remodelling. In this study we analysed, if cardiac endothelial cells are a source of TGF bexpression under hypoxic conditions. Microvascular endothelial cells were isolated form rat hearts. Cells were exposed to hypoxia (Hx) for 2 h followed by 15, 30 and 45 minutes, and 1, 2, 4 and 24 h reoxygenation (Rx). A significant induction of the 70 kDa precursor protein of TGFb was found after 15 minutes, 1 and 24 h Rx (n=21, p<0.05). An increase in TGFb-mRNA expression to 125 ± 7 % was found after 2 h Hx/4 h Rx (n=9, p<0.05). In order to analyse, if TGFb expression results in an autocrine reaction in endothelial cells we analysed activation of SMAD transcription factors, which are classical signalling molecules of the TGFb superfamily. After 2 h Hx and 2h or 24 h Rx, induction of p-SMAD2 to 130 ± 15 % or 113 ± 4 % (n= 13, p<0.05) was seen. This induction was inhibited by incubation of endothelial cells with TGFb type I receptor blocker SB431542 (1 mM). Phosphorylation of SMAD isoforms 1 and 5 was not detected. Addition of supernatants from 2 h Hx/1 h Rx endothelial cell cultures to cultures of ventricular cardiomyocytes enhanced phosphorylation of SMAD- 2 in cardiomyocytes within 1.5 h to 112 ± 1 % (n=4, p<0.05). In conclusion, under hypoxic conditions endothelial cells release TGFb that can induce SMAD-2 signaling in endothelial cells as well as cardiomyocytes.