Testicular biopsy specimens with meiotic arrest during spermatogenesis were studied with light and electron microscopy. The meiotic arrest occurred constantly at the end of meiotic prophase. The first degenerative changes were seen in the nucleus as a condensation of the chromatin along the synaptionemal complexes in the late pachytene spermatocytes. Later degenerative changes could also be seen in the cytoplasm. These results seem to indicate that the meiotic arrest primarily affects the nucleus of the pachytene spermatocytes.