Recently, investigators have reported that heat shock proteins (HSPs) can protect isolated cells from cytotoxicity induced by two important mediators of sepsis: interleukin-1 and tumor necrosis factor. The present study was undertaken to examine the hypothesis that transient whole body hyperthermia could decrease mortality from subsequent challenge with gram-negative endotoxin. We demonstrate that heat pretreatments improved long-term survival fivefold in a mouse endotoxin model and this was correlated with the production of HSPs. There was a marked difference in individual organ expression of the inducible 72-kDa heat shock protein (HSP72). Heat treatments caused significant HSP72 formation in lung, liver, kidney, and small intestine, but much lesser formation in heart, brain, and abdominal wall muscle. Additional experiments demonstrated that the protective effect of hyperthermic treatments against an endotoxin challenge occurred early, i.e., 1 and 2 h after heating, was maximal at 12 h, and had significantly diminished by 48 h. The formation and decay of HSP72 demonstrated a time course that paralleled the survival curve from endotoxin challenge, thus suggesting a possible role for HSP72 in the protective effect. Surprisingly, and in contrast to studies reported in incubated cells, endotoxin alone did not cause significant formation of HSP72 in vivo.