Rats of Albino Oxford (AO) strain in our animal facility exhibit a longer average healthy life span than rats of Dark Agouit (DA) strain. Since chronic activation of macrophages contributes to chronic low level inflammation common in older age, elucidation of the changes in middle-aged rats could be useful in prevention of unbalanced inflammatory response in advanced age. We have analysed the phenotype of unelicited and thioglycollate-elicited peritoneal macrophages from young and middle-aged DA and AO rats and tested functions of these cells following stimulation with lipopolysaccharide (LPS) in vitro. Unelicited cells from middle-aged DA rats produced higher amounts of proinflammatory mediators interleukin-6 (IL-6) and nitric oxide (NO), but have a diminished response to LPS stimulation then cells from young rats, in spite of increased frequency of TLR4- and CD14-expressing mature macrophages. Injection of thioglycollate robustly increased overall cytokine production in young rats' macrophages, while diminishing their response to LPS stimulation. In middle-aged DA rats injection of thioglycollate diminished IL-6 production, but increased it in response to LPS stimulation. Quite the contrary to DA rats, the macrophages from middle-aged AO rats have released diminished levels of TNF-α and NO, whereas urea production was strongly increased, when compared to the macrophages from young rats. Although the thioglycollate injection has increased the proportion of CD86(+)MHCII(+) mature macrophages in young rats, and percentages of activated TLR4(+) macrophages in both age groups of AO rats, it has not affected the cytokine production in young rats' macrophages, and the TNF-α production in middle-aged rats' macrophages. Moreover, the injection of thioglycollate has robustly increased the production of urea in macrophages derived from both age groups of AO rats. Although middle-aged rats of both strains were healthy during experiment, differences between the inflammatory responses of peritoneal macrophages of middle-aged rats of these strains might be one of the contributing factors defining their health in their advanced age. Development of strategies for the prevention of undesirable inflammatory changes in the elderly would benefit from the prospective study of the middle-aged.