Haptoglobin is a glycoprotein involved in the acute phase response. Previously we reported that haptoglobin gene expression was up-regulated during dietary restriction in young female rats. The present study aimed at determining whether chronic dietary restriction affects haptoglobin blood levels through changing levels and/or activities of IL-6-related transcription factors STAT and C/EBP in the liver as is the case during the acute phase response. To this end, we compared a female Wistar rat model of 50% 6-week-long dietary restriction with the standard laboratory model for the acute phase response induced by turpentine administration. During the turpentine-induced acute phase response, the transitory 5.4-fold increase of rat haptoglobin expression was accompanied by a prominent rise of serum IL-6 concentration and the increased binding of STAT3 and 35kD C/EBPbeta/LAP transcription factors to the haptoglobin gene hormone-responsive element. Results obtained after immunoblotting and DNA affinity chromatography (using hormone-responsive element) suggest that the stable 1.7-fold increase of serum haptoglobin level during dietary restriction was the result of increased amounts and activities of constitutive transcription factors C/EBPalpha and STAT5b, and to a smaller extent of STAT3. When dietary restriction rats were administered turpentine, a 8.7-fold increase in haptoglobin expression was followed by a considerable increase in the amount and hormone-responsive element binding activity of STAT3 but not 35kD C/EBPbeta/LAP. We concluded that haptoglobin gene up-regulation during chronic dietary restriction was regulated by different mechanisms than during the acute phase response, and that it depended on the amount(s) and activit(ies) of transcription factor(s) that characterize low-grade inflammatory conditions.