White oak heartwood samples were exposed to 650, 950 and 1900 MRad of cobalt-60 gamma radiation. The holocellulose portion of the heartwood cell walls was degraded while the lignin percentage remained relatively unchanged. Tangential vessel diameter, ray cell length, and length and width of intervessel pits increased upon exposure while tangential vessel-wall thickness, ray cell double-wall thickness, and latewood fiber double-wall thickness decreased