We studied the short-term effect of silicon carbide whisker (SiCW) in vivo by instillation and inhalation to the rat lung. SiCW was instilled low dose (2 mg/0.5 ml saline) or high dose (10 mg/ 0.5 ml) intratracheally into the lungs of 25 rats. SiCW was also inhaled to another 25 rats at the average concentration of 10.4 mg/m3 for 1 month. In instillation study, the lung had focal alveolitis with the destruction of alveolar wall especially at 3 days after the instillation, and the lesion remained as an aggregated foci of SiCW at 6 months. The 'inflammation-score' of the instilled group by point counting method of the specimen correspondingly decreased gradually. In inhalation group, a minimum inflammatory change was observed. Collagen deposition in the aggregated foci of SiCW with accumulated alveolar macrophages and neutrophils was not progressive during the observed period. These findings suggest that SiCW may cause a minor effect to the rat lung in 6 months after exposure.