For a chronic aerosol inhalation study of room-aged cigarette sidestream smoke and diesel engine exhaust, nose-only exposure chambers that enable the simultaneous exposure of up to 200 rats per chamber were required. The nose-only exposure mode was preferred because it prevents the deposition of aerosol particles on the skin and fur of the animals as far as possible, thereby minimizing the uptake of aerosol particles by grooming (Mauderly et al., 1989, Hausmann et al., 1998). In addition, it prevents the re-breathing of the exhaled test aerosol. Because of the large number of rats to be exposed simultaneously for 6 hours per day on a limited floor space, none of the commercially available nose-only flow-past chambers were suitable (Cannon et al., 1983, Pauluhn, 1994, Jaeger, 1994). Here, we describe a newly developed nose-only flow-past inhalation exposure chamber with an improved design that has the capacity for the simultaneous exposure of 198 rats, but which requires only a minimum of laboratory space (70 x 120 cm). To validate the exposure chamber, room-aged cigarette sidestream smoke and diesel engine exhaust were used as test aerosols. The aerosol within the chamber was uniformly distributed to all exposure positions within 6 s at the applied flow rate. The total particulate matter loss between the first and the last exposure row varied within the relative standard deviation of the aerosol concentrations determined at different sites within the entire exposure system. The particle size distribution of the two aerosols within the exposure chamber was in good agreement with literature values. The air supply was sufficient without inducing a draft in the breathing zone of the rats. A major advantage of this nose-only flow-past inhalation exposure chamber over other chambers is the flow direction and velocity in the breathing zone. Other flow-past exposure chambers blow the aerosol at a relatively high velocity directly into the face of the animals. Thus, the aerosol mixes with the exhaled air of the animals and may be partially re-breathed (Cannon et al., 1983, Pauluhn, 1994, Jaeger, 1994). In the exposure chamber described here, the aerosol flow passes the nose of the animal without changing major flow direction, thus preventing the re-breathing of exhaled air. At an exposure chamber inlet flow rate of 1 l/(rat x min), the aerosol velocity in the breathing zone of the rat is approximately 0.1 m/s. This is lower than the maximum air velocity tolerated in the work place (0.2 m/s) and protects the rats from the effects of irritative air draft, e.g., tearing. This newly developed nose-only flow-past inhalation exposure chamber was proven to perform well during a two-year inhalation study with rats. The exposure chamber design was submitted for invention disclosure and has been published under appl. No. 08250701.3 by the European patent office.