Objective: To study the effects of halothane anesthesia in mice not undergoing surgery on elements of the inflammatory and stress response; this involved assessment of the phagocytic activity and respiratory burst of peritoneal macrophages as well as plasma corticosterone levels and peripheral leukocyte counts. Methods: There were 2 experimental groups, i.e. mice anesthetized with halothane 1.5% in oxygen for 40 min and a control group of mice subjected to the same manipulations but no anesthesia. At the end of the anesthetic or sham procedure, peritoneal macrophages were evaluated for phagocytic and lytic activity after an immune challenge and spontaneous respiratory burst (chemoluminiscence). Plasma corticosterone and leukocyte counts in peripheral blood were evaluated as indicators of the stress response. Results: In halothane-anesthetized mice, increased numbers and activity of phagocytic cells were found, with regard to the number of ingested and digested particles, compared to the nonanesthetized group. The ex vivo peritoneal macrophage respiratory burst without antigenic stimulation also showed a higher response in anesthetized mice compared with the nonanesthetized controls. Halothane administration did not alter corticosterone levels. Treated and control mice displayed similar leukocyte profiles in peripheral blood, except for lower lymphocyte counts in the controls compared to the halothane group. Typical correlation between corticosterone and leukocyte subsets, together with a high positive correlation between plasma corticosterone and phagocytic cell counts, were found only in the control group. Conclusion: Halothane anesthesia might have beneficial effects on the inflammatory response mediated by phagocytes, namely the activity and efficiency of peritoneal macrophages, in a setting where plasma corticosterone and peripheral leukocyte counts were not affected.