Two cultivars of corn ( Zea mays L., cv. ‘WF9 × 38-11’ and ‘Golden Bantam’) were treated at different developmental stages with 60 Co gamma radiation at different exposures and rates. When given an acute exposure in the seedling stage, Golden Bantam was more sensitive than WF9 X 38-11 as measured by a reduction in survival and grain yield. A plot of the effect of exposure rate on the level of exposure required to eliminate grain production (YD 100 ) gave parallel curves for the the two cultivars and showed dependence of YD 100 on exposure rate below 16 R/min. Additional experiments demonstrated that WF9 × 38-11 was affected more severely by acute irradiation during tassel and ear primordia initiation than during the vegetative stage as measured by a reduction in plant height and grain yield. When chronic irradiation was used, WF9 × 38-11 plants exposed during the vegetative and gametogenic stages were affected more severely than those exposed during the embryogenic stage as measured by a reduction in height and grain yield.