Hydrogen peroxide $(H_2O_2)$ was detected cytochemically, via transmission electron microscopy (TEM), in pumpkin tissues exposed to high-dose gamma ray. Its reaction with cerium chloride produced electron-dense precipitates of cerium perhydroxides. Their patterns of deposition in the tissues of both control plants and those irradiated with gamma ray (PIG) were typically found in the plasma membranes and cell walls. However, gamma irradiation remarkably increased the intensities of cerium perhydroxide deposits (CPDs) in the plasma membranes and cell walls for all tissue types, but especially the leaves. The only exception was for vessels in the cotyledons. After gamma irradiation, the $H_2O_2$ content in all tissues was higher than in the control samples, except for the cotyledons of PIG, where the $H_2O_2$ content was lower than for all others. The increased appearance of CPDs may have been due to the enhancement of $H_2O_2$ accumulation by gamma radiation. This accumulation also varied according to the cell or tissue type examined.