We hypothesized that the systemic inflammatory response undergoes two consecutive stages, each characterized by different nonspecific sickness patterns. To test this hypothesis, we studied thermal, nociceptive, and motor responses to lipopolysaccharide (LPS) in 43 unanesthetized, habituated, and lightly restrained male Wistar rats previously implanted with a catheter in the jugular vein. Escherichia coli LPS was injected intravenously in a dose of 0, 0.1, 1, 10, 100, or 1,000 micrograms/kg. Colonic temperature (Tc) was measured with a thermocouple. Changes in nociception were assessed by tail flick latency (TFL) to a noxious heat stimulus. Motor activity was evaluated using an observation-based activity score (AS). The two lowest doses were apyrogenic. The next dose induced a monophasic fever with a maximal Tc rise of 0.9 +/- 0.2 degrees C at 108 +/- 11 min post-LPS. The next two higher doses caused biphasic fevers with the first and second peaks of 0.7 +/- 0.1 and 1.4 +/- 0.1 degrees C (10 micrograms/kg) and 0.7 +/- 0.1 and 1.4 +/- 0.2 degrees C (100 micrograms/kg) occurring at 60 +/- 6 and 165 +/- 17 min and at 45 +/- 3 and 141 +/- 6 min, respectively. The highest dose of LPS resulted in a Tc fall (nadir, -0.6 +/- 0.1 degree C at 83 +/- 6 min). Two different sickness patterns were exhibited. The first (high Tc, low TFL and high AS) occurred during the monophasic fever and the first (early) phase of the biphasic fevers, and it was termed the early phase syndrome. The second pattern (high or low Tc, high TFL, and low AS) developed during the second (late) phase of the biphasic fevers and LPS-hypothermia (endotoxin shock), and it was termed the late phase syndrome. Occurring at different stages of the systemic inflammatory response and developing through different coping patterns [fight/flight (energy expenditure) vs. depression/withdrawal (energy conservation)], the two syndromes represent two different types of adaptation to infection and have different biological significance. Viewing sickness as a dynamic entity is justified clinically. Such a dynamic approach to the problem resolves several contradictions in the current concept of sickness.