This study was initiated to assess the pulmonary toxicity of a polyolefin fiber composed of polypropylene in male Fischer 344 rats after 90 days of inhalation exposure. To increase fiber respirability in the rodent, polypropylene fibers were size-selected before aerosolization to have a geometric mean diameter of 1.6 microns (46% < 1 micron) and a geometric mean length of 30.3 microns. Three groups of animals were exposed in nose-only inhalation chambers, 6 hr/day, 5 days/week, for 90 days to 15, 30, or 60 mg/m3 of polypropylene, or filtered air (negative control). Microscopic examination of the polypropylene fiber-exposed lungs revealed that, at all time points examined in the study, there was a dose-dependent increase in pulmonary macrophages. These minimal or mild increases in cellularity appeared to be reversible, especially at the lower doses 30 days post exposure. No fibrosis was observed in any of the groups. A strong correlation was found between the external exposure concentration, the time of exposure, and the lung fiber burden. The number of partially degraded (segmented) fibers within the lung increased with the exposure concentration and period of exposure, as well as with the period of recovery after termination of exposure at 90 days. Fibers were recovered from exposed lungs using a hypochlorite digestion technique.