Introduction and aims - Nanoparticles of titanium dioxide are suspected neurotoxic agents and have numerous applications possibly resulting in human exposure by several ways including inhalation. In the present work, rats were exposed to spherical TiO2 nanoparticles of two different sizes by the intratracheal route. It was investigated how the neuro-functional alterations, detected by electrophysiological and behavioral methods, were related to the concentration of Ti in the tissue samples and what the influence of the size of the NPs was. Materials and methods - Rats (young adult Wistar males, 10/group) were exposed to TiO2 nanoparticles of ca. 10 and 100 nm diameter (suspension medium: neutral PBS with 1% hydroxyethyl cellulose) by intratracheal instillation in 5 and 18 mg/kg b.w. dose; 5 days per week for 6 weeks. Controls were instilled with saline, and vehicle controls, with the suspension medium. To see general toxicity, body weight was checked daily, and organ weights were measured at the end of experiment. Grip strength test, to assess motor function damage, was done before and after the 6-week treatment. Finally, the rats were anesthetized with urethane, spontaneous cortical activity and sensory evoked potentials were recorded, then the rats were dissected and tissue samples were taken for Ti level measurement. Results - Body weight gain indicated no general toxicity, and no significant change in the relative organ weights, except that of the lungs, was seen. However, change of time-to-fall in the grip strength test, and latency of cortical evoked potentials, were altered in the treated groups, indicating functional damage. Correlation of these alterations with the cortical Ti level was dissimilar for the two sizes of nanoparticles. Conclusion - The results provided further support to the functional neurotoxicity of TiO2 nanoparticles. The exact role of particle size, and the mechanisms involved, remain to be elucidated.