<p><b>OBJECTIVE</b>To investigate the effects of different concentrations of norepinephrine (NE) on proliferation and apoptosis of cultured cardiac fibroblasts (CFBs) from neonatal mice and to elucidate related mechanisms.</p><p><b>METHODS</b>CFBs of Sprague-Dawley (SD) rats were isolated and cultured and divided into normal control group and different concentration of NE intervention groups (0.1, 1, 10, 50, and 100 µmol/L). Water soluble tetrazolium-1 (WST-1) assay was carried out to detect the viability of CFBs. Morphology of apoptosis cells was evaluated by fluorescence microscope with terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. The expressions of collagen I, collagen III, pro-oncogene c-myc in CFBs were detected by reverse transcription-polymerase chain reaction (RT-PCR). The phospho-mitogen activated protein kinase (p-p38MAPK) and caspase3 protein levels were examined by Western blot.</p><p><b>RESULTS</b>Proliferation was significantly increased in 1 µmol/L and 10 µmol/L groups compared with the normal control group (1.05 ± 0.05 and 1.09 ± 0.02 vs. 1.00 ± 0.03, all P < 0.05).CFBs apoptosis was significantly enhanced in 50 µmol/L and 100 µmol/L groups ((22.69 ± 2.18)% and (36.40 ± 6.80)% vs.(4.50 ± 1.08)%, all P < 0.05). Expression of Collagen I peaked in 10 µmol/L group, expression of collagen III and c-myc increased dose-dependently in proportion to increasing NE concentrations (all P < 0.05 vs. control group). The expression of p-p38MAPK and caspase3 was also significantly upregulated in a dose-dependent manner in NE groups (all P < 0.05 vs. control group).</p><p><b>CONCLUSIONS</b>Low concentration NE induces CFBs proliferation and high concentration NE promotes CFBs apoptosis. p38MAPK phosphorylation may be a major mediator of NE-induced effects on CFBs.</p>