The morphology of the mouse lens was studied by light and electron microscopy between 4 and 30 months after graded single doses of photons or charged particle exposure. Four-month-old mice were given upper body or head only doses of 220 kVp x-rays or to 40Ar (570 MeV/amu), 40Ar(500 MeV/amu), 20Ne(470 MeV/amu) and 12C (400 MeV/amu) particles. The lenses examined in this pilot study were largely from mice dedicated to other designed experiments; nor all radiation conditions are represented at each sample time. At 4 months after 0.05, 0.3 or 0.9 Gy 20Ne radiation, the micronuclei appeared in the epithelial cells at the bow region. The anterior fibers showed swollen cytoplasm containing amphorous materials after 0.3 or 0.9 Gy 20Ne exposure. The posterior fibers were relatively normal. The results at 15 months after irradiation indicated heavy charged particles were more effective than x-rays for inducing morphological changes; however there were no qualitative differences in damage produced by heavy charged particles and x-rays. Characteristic changes after irradiation included abnormal bow patterns, posterior and anterior migration of nucleated cells in the cortex, swelling of fibers and the development of swirls of epithelium at the anterior pole. The results also indicated that the degree of degenerative changes in lens fibers increases with increasing radiation dose and sampling time. The radiation-induced injury to lens fibers also showed a relationship to linear energy transfer (LET). The qualitative observations at 24 months indicated that low energy 40Ar (600 KeV/micron) is less effective than high energy 40Ar particles (100 KeV/micron)1 and radiation-induced injury in the lens fibers shows an LET-dependence when the ionizing density is less than 100 KeV/micron. At 30 months after heavy charged particle irradiation, the changes in control lens were the swollen fibers, while in the 40Ar-irradiated animals, aberrant fibers with abnormal nuclei, amorphous materials and debris were observed.