The objective of this inhalation study was to determine and evaluate the potential toxic effects of dicyclopentadiene vapor in mice exposed for 13-weeks of repeated inhalation. Four groups, each consisting of 45 male and 45 female B6C3F1 mice, were exposed to dicyclopentadiene vapor by inhalation 6 h/day, 5 days/week, for 13 weeks (64 exposures) at targeted concentrations of 0, 1.0, 5.0, or 50 ppm. An assessment of toxicity is carried out after 2, 6, and 13 weeks of inhalation exposures. Additionally, animals were evaluated during the recovery period of 4 or 13 weeks after the last exposure. Observations and measurements to assess toxicity include clinical observations, body weight, organ weights, serum chemistry, and hematologic, ophthalmologic, gross pathologic, and histologic evaluations. The only systemic effects observed were a few statistically significant changes in organ weights; but these were considered spurious in nature. Ten male and nine female mice in the highest exposure group died during the study, while no more than two mice died in any other group. However, the excess mortality was without an apparent etiology or association to exposure and was attributed to pulmonary congestion as a consequence of pulmonary irritation. Under the conditions of this study, these data demonstrated that in the absence of overt systemic toxicity, respiratory congestion has the predominant effect at the exposure levels of 50 ppm. This observation contributes to the Globally Harmonized System harmonized hazard classification of Single Target Organ Toxicity - Single Exposure (H335, may cause respiratory irritation) for this substance.