TNF-alpha gene expression is regulated at transcriptional and post-transcriptional levels in mouse macrophages. The post-transcriptional regulation is mediated by the AU-rich element (ARE) located in the TNF-alpha mRNA 3' untranslated region (UTR), which controls its translation and stability. In resting macrophages, the ARE represses TNF-alpha mRNA translation. Activation of macrophages with various agents [for example lipopolysaccharide (LPS), viruses] results in translational derepression, leading to the production of high levels of TNF-alpha. TNF-alpha ARE has also been shown to confer mRNA instability as its deletion from the mouse genome leads to an increase in the TNF-alpha mRNA half-life [Kontoyiannis, D., Pasparakis, M., Pizzaro, T., Cominelli, F. & Kollias, G. (1999) Immunity 10, 387-398]. In this study, we measured the half-life as well as the poly(A) tail length of TNF-alpha mRNA in the course of macrophage activation by LPS. We report that TNF-alpha mRNA is short lived even in conditions of maximal TNF-alpha synthesis. Moreover, TNF-alpha mRNA is hypoadenylated in a constitutive manner. These results reveal that TNF-alpha mRNA rapid turnover does not constitute a regulatory step of TNF-alpha biosynthesis in macrophages and that TNF-alpha mRNA translational activation upon LPS stimulation is not accompanied by a change of poly(A) tail length.