Locally irradiated bone marrow was uniformly depressed during the first week after radiation doses of 2000 to 10,000 rads. The early appearance of hemopoietic regeneration between 7 and 14 days at all dose levels suggested an influx of stem cells from unirradiated sites. Between 2 and 6 months post-irradiation a second wave of hemopoietic depression occurred at all dose levels and was correlated with the disappearance of the sinusoidal microcirculation. Between 6 and 12 months diffuse hemopoietic regeneration occurred only at the 2000 rad level and could be correlated with regeneration of the sinusoidal microcirculation. No significant hemopoiesis or sinusoidal regeneration was observed at the higher dose levels after 6 months or 1 year. Although large vascular channels developed, indicating circulation through the marrow cavity, the characteristic and essential anatomy of the microcirculation was not restored.