The time course of changes in the levels of acute-phase-reactant (APR) mRNAs in different tissues of rats with a 10% or a 60% total-body-surface-area (TBSA) burn and the relationship between the induction of APRs and the host's tolerance to thermal injury were studied. The acute phase response in a LPS-induced inflammation model and a burn-plus-LPS model were compared. The results of this study indicated that (1) the major site of APR synthesis is the liver; (2) even a small surface burn injury can elicit a rapid acute phase response, but the intensity of APR expression increases with the severity of the burn; (3) the down regulation of albumin mRNA, which is characteristic of the acute phase response, does not occur even though transferrin (Trf) mRNA levels are significantly decreased; (4) the resistant strain of inbred rats showed higher levels of alpha 1-antitrypsin (AT) mRNA before and after burn injury, indicating its contribution to the host's tolerance to thermal injury; (5) the increases in alpha 1-acid glycoprotein (AGP) and AT expressions are limited in the burn-plus-LPS rat model compared with either the burn model or LPS-stimulated model alone.