An acute (4-hr) and a sub-acute (4-wk) inhalation toxicity study of germanium metal powder (purity 99.8%, mean particle size 2.0-2.4 microns) were carried out in young adult Wistar rats. Exposure of five male and five female rats to 3.86 or 5.34 g/m3 for 4 hr resulted in the death of one rat at each exposure level. Four groups of five male and five female rats were exposed to 0, 9.9, 65.1 or 251.4 mg/m3 for 6 hr/day, 5 days/wk for 30 days. Two additional (recovery) groups of five male and five female rats exposed to 0 or 251.4 mg/m3 were kept untreated for 31 days after exposure. At the end of the treatment period, fasting blood glucose was decreased in males exposed to the high concentration. In females of this group, blood creatinine and urea levels, and urine volumes were increased, but urine density was decreased. Increased blood creatinine levels and urine volume and decreased urine density were also observed in females exposed to 65.1 mg/m3. The absolute and relative lung weights were increased in rats in the mid-and high-concentration groups. Histopathological examination revealed: accumulation of particulate material in the lungs of all treated groups, accumulation of alveolar macrophages in the mid- and high-concentration groups, and alveolitis mainly in the high-concentration group. After the 4-wk recovery period, urine volume was increased in males that had been exposed to germanium. In exposed rats of both sexes, lung weights were still increased and histopathological changes were present, but to a lesser extent than at the end of the exposure period. It was concluded that the 4-hr LC50 value of germanium metal powder in rats is greater than 5.34 g/m3. The no-adverse-effect level in the 4-wk study was 9.9 mg/m3 air.