C-reactive protein (CRP) is a typical acute-phase protein found in many animals. Several stimuli, including IL-6 produced by macrophages localized around the site of tissue injury and/or bacterial infection, induce CRP synthesis in liver. One of the authors (W.N.) reported previously that sex hormones also affect the serum CRP concentration in rats. However, little is known about the process of CRP production, including time-dependent changes in the CRP mRNA level in liver. In the present study, we observed time-dependent changes in the serum concentration of CRP and the CRP mRNA level in liver after the injection of turpentine-oil or estradiol-17 beta in rats (Wistar strain). After turpentine-oil injection, CRP mRNA increased, followed by an increase in the serum CRP concentration, indicating that inflammation enhances transcription of the CRP gene. In contrast, upon estradiol-17 beta administration, the serum CRP concentration decreased without any decline in the CRP mRNA level in liver. The latter finding suggests that rats may have an alternative mechanism for regulating the serum concentration of CRP.