Macrophages previously exposed to bacterial lipopolysaccharide (LPS) develop a "tolerant" response with decreased extracellular signal-regulated kinase (ERK) activation in response to LPS rechallenge. Prior work using 21-hour LPS pretreatment showed that 100 ng/mL of LPS-inhibited tumor necrosis factor (TNF) release, whereas very low dose LPS (1 ng/mL) augmented TNF release. Endotoxin tolerance was also associated with alterations in activation of ERK and p38 kinase when cells were restimulated with LPS. We hypothesized that the interval after pretreatment, before LPS rechallenge, modulates macrophage response to LPS.RAW 264.7 macrophage-like cells were pretreated for 4 hours in 0 ng/mL (none), 1 ng/mL, 10 ng/mL, or 100 ng/mL of Escherichia coli 0111:B4 LPS. After 4 hour pretreatment, medium was discarded. Cells were rechallenged immediately or 21 hours later with 0 ng/mL, 1 ng/mL, 10 ng/mL, or 100 ng/mL LPS. Supernatant TNF secretion at 3 hour was measured using enzyme-linked immunosorbent assay. Active phospho-ERK was examined by Western blot using specific monoclonal antibodies 30 minutes after LPS rechallenge. Statistical analysis by chi and student's t test.When macrophages were pretreated for 4 hour and incubated overnight (21-hour interval) 1 ng/mL of LPS augmented and 100 ng/mL inhibited TNF release with LPS rechallenge. In contrast, with immediate rechallenge, we saw additive effects with 100 ng/mL LPS and no difference with 1 ng/mL LPS versus no pretreatment. Western blot revealed that even with immediate rechallenge "tolerant" macrophages were unable to activate ERK.A short LPS exposure is sufficient to induce alterations in ERK activation in macrophages, but longer intervals are required to express altered cytokine release. In conjunction with other recent findings, these results suggest that both pretreatment dose and interval modulate macrophage responsiveness to LPS rechallenge.