The physiological agonist norepinephrine (NE) promotes cell proliferation of brown preadipocytes during the process of tissue recruitment. In a primary culture system, cAMP was shown to mediate these adrenergic effects in the presence of serum. In the present study, we have demonstrated that in con- trast to other systems where the cAMP mitogenic effect requires the syner- gistic action of growth factors, especially insulin/IGF, the cAMP effect in brown preadipocytes is independent of serum and insulin. Furthermore, we showed that the increase in total DNA resulted from forskolin stimulation of cell proliferation, in addition to the anti-apoptosis effect reported previously. Protein kinase A, rather than Epac, mediated the cAMP mitogenic effect. In a further attempt to elucidate the cell signalling pathways involved in this process, we found that both mTOR complexes and the ERK 1/2 family of MAPK, two cell proliferation hallmarks, are indeed activated by cAMP. However, mTOR inhibition with rapamycin only caused partial inhibition of cell proliferation, and Ku-0063794, another inhibitor of mTOR, showed no inhibitory effect on cAMP-stimulated cell proliferation. The involvement of ERK1/2 was tested with pharmacological inhibitors of ERK1/2. PD98059 brought about a significant inhibitory effect, whereas U0126 did not have any inhibitory effect on cAMP-stimulated cell proliferation, even though ERK1/2 phosphorylation was equally well abolished by both inhibitors. Taken together, these results suggest that in brown preadipocytes under se- rum-free conditions, PKA, rather than Epac, mediated the cAMP mitogenic effect through activation of a pathway sensitive to PD98059 but inde- pendently of PI3K, mTOR complexes and the ERK1/2 family of MAPK.