The use of nanotechnology is increasing exponentially, whereas the possible adverse health effects of engineered nanoparticles (NPs) are so far less known. Standardized mouse bioassay was used to study sensory and pulmonary irritation, airflow limitation, and inflammation potency of nanosized TiO(2). Single exposure (0.5 h) to in situ generated TiO(2) (primary particle size 20 nm; geometric mean diameters of 91, 113, and 130 nm at mass concentrations of 8, 20, and 30 mg/m(3), respectively; crystal phase anatase + brookite (3:1)) caused airflow limitation in the conducting airways at each studied exposure concentration, which was shown as a reduction in expiratory flow, being at the lowest 73% of baseline. The response was not dose dependent. Repeated exposures (altogether 16 h, 1 h/day, 4 days/week for 4 weeks) to TiO(2) at mass concentration of 30 mg/m(3) caused as intense airflow limitation effect as the single exposures, and the extent of the responses stayed about the same along the exposure days. Sensory irritation was fairly minor. Pulmonary irritation was more pronounced during the latter part of the repeated exposures compared to the single exposures and the beginning of the repeated exposures. Sensory and pulmonary irritation were observed also in the control group, and, therefore, reaction by-products (NO(2) and C(3)H(6)) may have contributed to the irritation effects. TiO(2) NPs accumulated mainly in the pulmonary macrophages, and they did not cause nasal or pulmonary inflammation. In conclusion, the irritation and inflammation potencies of studied TiO(2) seemed to be low.