Using a 500 kHz radiofrequency electromagnetic heating system, the effects of localized current field hyperthermia in normal rabbit eyes were examined. A specially designed scleral plaque placed on normal rabbit eyes was heated to temperatures of 43 degrees C, 45 degrees C, and 47 degrees C for a period of 45 min. The effects of hyperthermia were monitored by clinical examination, fluorescein angiography, electroretinography and histopathology. A graded effect with increasing temperature was found at the lower temperature, and it was confined to the treatment field. At 47 degrees C the electroretinogram was extinguished due to diffuse photoreceptor damage outside the treatment field, as demonstrated by histopathology and electron microscopy. This study indicates that hyperthermia at 45 degrees C for 45 min is the maximum allowable temperature without causing diffuse retinal damage in the normal rabbit eye.