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Ambient particulate pollution is associated with adverse health effects in epidemiological studies of the elderly with cardiopulmonary diseases. We hypothesize that ultrafine particles (UFP) contribute to these effects, especially when they are freshly generated and occur at high number concentrations. Studies to determine adverse effects have been performed using laboratory-generated surrogates, diluted exhaust from stationary engines, or concentrated ambient UFPs. Methodological difficulties exist with such experiments, and questions remain about how well these particles model those found in ambient air. Freshly generated UFPs are present at high concentrations on highways and vehicle passengers are directly exposed to them. We wished to expose rats to these UFPs to test their potential to cause effects. Since such exposures have not been done before, one objective of our study was to demonstrate the feasibility of an on-road exposure study. Secondly, we wished to determine if there are significant exposure-related effects in aged, compromised rats. Old rats (21-mo F-344) were exposed directly on highways to either the aerosol (<1 microm)/gas phase, gas phase only, or filtered air using an on-road exposure system. Some rats were pretreated with a low dose of inhaled endotoxin or with instilled influenza virus to induce lung inflammation. The exposures in compartmentalized whole-body chambers consisted of 6-h driving periods on I-90 between Rochester and Buffalo once or 3 days in a row. Endpoints related to lung inflammation, inflammatory cell activation, and acute-phase responses were measured after exposure. The on-road exposure system did not affect measured endpoints in filtered air-exposed rats, indicating that it was well tolerated by them. We observed the expected increases in response (inflammation, inflammatory cell activation) to the priming agents. We also found a significant particle-associated increase in plasma endothelin-2, suggesting alterations in vascular endothelial cell activation. In addition, we observed main effects of particles related to the acute-phase response and inflammatory-cell activation. Interactions between on-road particles and the priming agents were also found. These results suggest that exposures to on-road particle mixtures have effects on the pulmonary and cardiovascular system in compromised, old rats. Furthermore, they demonstrate that on-road exposures are feasible and could be performed in future studies with more continuous particle exposures.