During the last decades, the prevalence of allergy has increased worldwide. Allergic rhinitis ("hay fever") and asthma are two of the most common allergic diseases. A possible cause for increased allergy to pollen is air pollution. The increase of industrialization and the number of diesel engines associated with diesel exhaust particles (DEP) in the air parallel the increase in allergic airway diseases. To investigate the adjuvant effect of DEP in pollen allergy, Brown Norway (BN) rats were sensitized intranasally or intratracheally with timothy grass pollen (Phleum pratense) with or without DEP (3 mg/ml). Intranasal sensitization (200 microl, 10 mg/ml) was performed daily for 5 consecutive days and intratracheal sensitization (200 microl, 10 mg/ml) was performed once. Challenge with pollen was performed at day 21 similarly to the sensitization protocol. Blood samples were taken at day 28 after the first sensitization. The binding of DEP to pollen grains was studied by scanning electron microscopy and the inflammatory response in the lung was studied by light microscopy. Immunoglobulin E (IgE) and IgG(1) responses against pollen grains were measured by digoxigenin (DIG) enzyme-linked immunosorbent assay (ELISA). Scanning electron microscopy revealed a mixture of free DEP and DEP associated with pollen grains. Both intranasal and intratracheal routes of administration of pollen grains induced inflammatory reactions in the lung with an influx of macrophages, eosinophilic granulocytes, and granuloma formation. Pollen grains were localized in the alveoli after both intranasal and intratracheal administration and were surrounded by macrophages. The number and localization of pollen grains were similar for both routes of administration. After coexposure with DEP, DEP-loaded macrophages were found around the pollen. Localization, inflammatory reaction, and integrity of pollen were similar to those seen without DEP. At day 28, specific IgE and IgG(1) antibodies were found in serum of rats immunized intranasally or intratracheally. IgE antibody response was higher in rats immunized with pollen grains and DEP than in rats immunized with pollen only (dilution mean +/- SEM: 59.4 +/- 4.6 vs. 27 +/- 5.1). The IgG(1) antibody response was much higher compared to the IgE response (factor of 10(4)), but the level of IgG(1) antibodies was only slightly increased by DEP (dilution mean +/- SEM: 24.2 +/- 2.0 x 10(4) vs. 16.1 +/- 2.1 x 10(4)). In conclusion, the intranasal application of pollen in the BN rat is a suitable and elegant method to evoke inflammatory reactions in the lung and pollen-specific IgE responses measured by DIG ELISA. Finally, this model gives similar results on adjuvant activity of DEP found in the ovalbumin models presented previously.