To evaluate the safety of rock wool (RW fibers), we examined the biopersistence of a RW sample in the lungs of rats, based on the changes of fiber number and fiber size in terms of length and width, by a nose-only inhalation exposure study. Twenty male Fischer 344 rats (6–10 weeks old) were exposed to RW fibers at a concentration of 70 (21) fiber/m³ and 30 (6.6) mg/m³, arithmetic mean (geometric standard deviation), continuously for 3 h daily for five consecutive days. Five rats each were sacrificed shortly and at 1, 2, and 4 weeks after exposure, and their lung tissues were ashed by a low-temperature plasma-asher. Then, the numbers and sizes of fibers in the ashed samples were determined using phase-contrast microscope and computed image analyzer. The fiber numbers in the lungs 4 weeks after exposure significantly decreased from the baseline value, i.e., shortly after exposure ( P P