OBJECTIVE AND DESIGN: To quantify the number of mast cells in the skin of rabbits during initiation and progression of the local Shwartzman reaction. MATERIALS: Thirty New Zealand rabbits were divided in two groups (n = 15/group). One group was subjected to the Shwartzman reaction and the other group served as control. Subsequently, animals were further subdivided in six groups of five animals each according to time of euthanasia. TREATMENTS: The local Shwartzman reaction was induced by two inoculations of Salmonella typhimurium lipopolysaccharide. Preparatory inoculation was given intradermally and, 24 h later, the provocative injection was administered intravenously. Controls were subjected to the same procedure but received saline. After provocative injection animals were killed at 1, 8, and 15 days. METHODS: Skin samples were fixed in Carnoy's solution and mast cells were identified employing a low pH toluidine blue stain. Numbers of mast cells were determined per square millimetre and, subsequently, those cells degranulated were identified and quantified to obtain absolute values. A Student's t test was initially used to compare Shwartzman versus controls at each time point. Subsequently, an ANOVA test employing a factorial experiment was used to assess a possible interaction between time of euthanasia and treatments. RESULTS: The values were transformed (natural logarithms) for appropriate statistical comparisons. Independent comparisons at each time point showed that Shwartzman groups had higher numbers of mast cells than controls at 1 and 8 days, but not at 15 days (5.71 +/- 1.00 Vs. 2.40 +/- 0.58, P < 0.005; 3.77 +/- 0.90 Vs. 2.33 +/- 0.56, P < 0. 025, and 2.61 +/- 0.25 Vs. 2.39 +/- 0.39, P > 0.05, respectively). Degranulated cells were numerous in Shwartzman groups, particularly at day 1 (3.48 +/- 0.78) and less obvious at day 8 (0.72 +/- 0.50), but were scarce by day 15 (-0.67 +/- 0.99) as well as controls (-0.68 +/- 0.91). The factorial experiment demonstrated that the Shwartzman reaction and time of euthanasia were independently significant (P < 0.005) but their interaction at day 1 was the major contributor (P < 0.005). Tukey's w pairwise comparisons of means confirmed that the Shwartzman group killed at day 1 was significantly different from the others (P < 0.05). CONCLUSIONS: Mast cells significantly increase in the early stages of the local Shwartzman reaction. Thus, mast cells are a highly dynamic cell population, which have a prominent role during the acute phase of this lipopolysaccharide-induced inflammatory reaction but not during healing.