Optic and proximate tissues of New Zealand white rabbits were exposed to the Bragg plateau regions of 530 MeV/amu Ar ions and 365 MeV/amu Ne ions and also to 60Co gamma rays. The linear energy transfers (LET infinity s) for the radiations were 90 +/- 5, 35 +/- 3 and 0.3 keV/micrometer, respectively. Single acute exposures to all three radiations caused only limited changes in the electroretinogram (ERG) until a critical dose was approached. At that dose, which was 45 +/- 3 Gy for all three types of radiation, a sudden loss in the b-wave of the ERG occurred. From less accurate measurements, it appeared that the response of the a-wave of the ERG was similar. The critical dose for ERG loss was reflected also in the rejoining of strand breaks in the DNA of retinal photoreceptor cells. Above the critical dose, the rejoining of DNA strand breaks was limited; below the critical dose, the rejoining of DNA strand breaks approached completion even though it took days to do so. In the latter respect, the response of photoreceptor cells to heavy-ion irradiation in situ is different from that reported for proliferating cells in culture.