The degree of hematopoietic depression and spontaneous recovery of depressed cells with 50% nitrous oxide inhalation for 6 or 12 hours were studied in 120 Sprague-Dawley rats. Immediately after, 1 day, 3 days and 1 week after nitrous oxide inhalation, precursor cells of granulocyte-monocyte and T lymphocyte in bone marrow and blood were sampled and cultured. After one week of culture period, the numbers of colony forming unit-granulocyte monocyte (CFU-GM) and colony forming unit-T lymphocyte (CFU-TL) were counted. There was no change in the number of colony immediately after inhalation of nitrous oxide, but was significant decrease in number of colony 1 day and 3 days after inhalation of nitrous oxide (p<0.001). One week after cessation of nitrous oxide inhalation, spontaneous recovery of number of colony developed without any treatment. To evaluate the protective effect of folinic acid (Rescuvolin) against hematopoietic depression, Rescuvolin 0.1 mg/0.3 ml were injected intraperitoneally at 12 hours and 1 hour before, 1 hour before and immediately after nitrous oxide inhalation, respectively. The data of each group were compared with that of the animals which have inhaled nitrous oxide for 6 hours without folinic acid. Folinic acid injected groups showed significant increase in numbers of colony (p<0.001) especially in 12 and 1 hour before nitrous oxide inhalation. In conclusion, the longer the duration of nitrous oxide inhalation, the faster and severer hematopoietic depression developed. The pretreatment of folinic acid may prevent the bone marrow depression by a long-term and repeated use of nitrous oxide. In clinical anesthesia, it is recommended to avoid to use nitrous oxide for the patients with bone marrow depressed disease such as leukemia and aplastic anemia.