Objective: We studied the ability of norepinephrine and of other catecholamines to affect the proliferation of cardiac fibroblasts isolated from adult rat hearts. Furthermore, we investigated the possible adrenergic receptor involved in this process. Methods: Norepinephrine (NE), phenylephrine (PE), isoproterenol (ISO), forskolin (FO), epidermal growth factor (EGF), platelet-derived growth factor AA (PDGF-AA) and specific inhibitors of the α₁-, α₂-, β₁- and β₂-adrenoceptors and of the protein kinase A (PKA) were applied to cardiac fibroblasts in culture. Cell number was measured by use of a Coulter Counter. Activation of the cAMP response element binding protein (CREB) was measured by Western blotting and subsequent use of a phospho-specific antibody. Activation of the p42- and the p44-mitogen activated protein kinase (p42/p44^MAPK) was assessed by detection of phosphorylation shifts and by incorporation of ³²P-labelled phosphate into myelin basic protein. Results: Fibroblasts isolated from hearts of adult rats were grown in 10% serum-containing media which induced an increase in cell number by 94%. After 48 h, treatment with 10 μM NE caused an even greater increase in cell number by 222%, i.e. another 128% (comitogenic effect). In contrast, NE alone had no effect on the growth of serum-deprived cells. EGF and PDGF-AA did not replace serum as the basic mitogen. After addition of NE to proliferating cells under serum conditions, there was a rapid, time-dependent significant activation of the p42/p44^MAPK and of CREB for up to 60 and 120 min, respectively. In both cases, the maximum of activation was reached after 5 min. Application of FO (0.1–20 μM) caused a strong activation of CREB, while no increase in the phosphorylation of the p42/p44^MAPK was detected. Treatment with 20 μM FO led to an identical increase in cell number as application of NE. Specific blockade of PKA with RpcAMPS prevented the activation of CREB and also the comitogenic effect of FO as well as of NE. The α- and β-adrenergic receptor blocker carvedilol (10 μM) normalized all NE-induced effects. Prazosin and yohimbine, inhibitors of α₁- and α₂-adrenoceptor activation, respectively, did not influence the NE-evoked increase in cell number. In contrast, the non-selective β-adrenoceptor blocker propranolol (1 μM) completely suppressed the comitogenic effect of NE. A similar effect was obtained with the specific β₂-adrenoceptor blocker ICI 118,551 (5 μM), while the β₁-adrenoceptor blocker metoprolol did not influence the increase in cell number. Conclusions: NE elicits a comitogenic effect on cultured rat cardiac fibroblasts which is prevented by β₂-adrenergic blockade. The activation of CREB contributes to the increase in proliferation. The p42/p44^MAPK which was also found to be activated by NE might as well be involved in the regulation of the comitogenic effect.