Activin A is a kind of pre-inflammatory factor that belongs to the transforming growth factor-beta (TGF-beta) superfamily. To investigate the effect and mechanism of activin A on the activities of mouse macrophages, the secretion of NO in the supernatant of cultured mouse peritoneal macrophages was examined by NO assay kit, and the expression of iNOS, ActRIIA and ARIP2 mRNA in mouse peritoneal macrophages was analyzed by RT-PCR. The results showed that activin A stimulated the secretion of NO and the expression of iNOS mRNA in non-activated mouse macrophages in a time- and dose-dependent manner. In contrast, activin A in the same concentration inhibited the secretion of NO in LPS-activated mouse macrophages in a dose-dependent manner. ActRIIA was highly expressed on macrophages, and activin A upregulated the ActRIIA mRNA expression in macrophages. Anti-ActRIIA antibody can block the secretion of NO from the macrophages stimulated by activin A. Furthermore, RT-PCR analysis revealed that activin A enhanced the ARIP2 mRNA expression in macrophages. These results indicated that Activin A may be a weak activator compared with LPS to mouse macrophages, and activin A may modulate the secretion of NO through ActRIIA-ARIP2 signal pathway in mouse macrophages.