The aim of this study was to compare the ability of ultrafine titanium dioxide (Uf-TiO2), ultra-fine nickel (Uf-Ni), and ultrafine cobalt (Uf-Co) to cause damage and stimulation to alveolar macrophages from rats of different ages. Wistar male rats (SPF), aged 2 mo (young) or 20 mo (old), were sacrificed prior to bronchoalveolar lavage (BAL). Alveolar macrophages (AM) from either young or old rats were incubated with Uf-Co or Uf-Ni at a final Concentration of 2.5, 5, 10, 20, and 40 µg/ml, respectively. Macrophages were also incubated with 40 µg/ml ultrafine metals for 12, 24, or 48 h before biochemical indices were evaluated in the supernatant. There were dose-related increases in lactate dehydrogenase (LDH) activity and tumor necrosis factor-alpha (TNF-α) in the supernatant of young and old macrophages after treatment with various doses of Uf-Co or Uf-Ni. Both Uf-Co and Uf-Ni caused severe macrophage damage, which was reflected in an increase in the relative LDH activity in supernatant. Furthermore, macrophages from old rats released significantly more TNF-α than macrophages from young rats, especially after 12-h treatment with Uf-Co. The study has confirmed that three ultrafine metals differ in their toxicity and stimulatory effects on rat alveolar macrophages. Uf-TiO2 was relatively inactive in these assays of toxicity. There were clear differences in susceptibility to these effects in macrophages from young or old rats, with old rats showing a substantially greater TNF-α response. The mechanism still remains to be elucidated, but our earlier studies implicated oxidative stress in the proinflammatory effects of these ultrafine metals. In addition, oxidative stress has been implicated in aging. Therefore our data are consistent with the concept that aging is accompanied by oxidative stress that renders the aged lung susceptible to the proinflammatory effects of ultrafines.