One-day and six-day old Sprague-Dawley rats were exposed in the far field to 2.45 GHz (cw) microwaves at 10 mW/cm2 for five consecutive days, 7 hours per day (SAR 2W/kg). Pups were euthenized one day after exposure and the cerebella processed for light and electron microscopy. Matching cerebellar sections and folia from irradiated and sham irradiated animals were examined. Light microscopic examination revealed the presence of small deeply-stained cells with hyperchromatic pyknotic nuclei within the external granular layer (EGL). The number of these pyknotic cells in the experimental animals was nearly twice that in the controls. The Nissl bodies in Purkinje cells were finely dispersed. In some experimental animals mononuclear cellular infiltration was demonstrated. Under the electron microscope the deeply-stained pyknotic small cells presented electron dense nuclei with clumped chromatin, extrusion or disintegration of the nucleus, ruptured nuclear membrane, and the vacuolization of the cytoplasm. Eventually these cells became phagocytosed by surrounding EGL cells. Most of the Purkinje cells of experimental animals showed small, disorderly arrays of rough endoplasmic reticulum (RER) instead of the typical orderly stacks of parallel arrays. These observations suggest that microwave radiation may interfere with early genesis of cerebellar microneurons and alter the metabolic status of Purkinje cells. However, this effect might be reversible.