The sizes and staining intensities of the nucleolus organizer regions (NORs) in human spermatogenesis were studied qualitatively and quantitatively with the silver (Ag)-staining technique. The Ag-stainability of the NORs is a measure of the transcriptional activity of the ribosomal RNA genes. Ag-stained NORs are present during the whole period of meiotic prophase up to pachytene and totally absent in the two meiotic metaphases. In the early spermatids the Ag-stainability of the NORs reappears and persists up to the beginning of the elongation phase of the nuclei. This pattern of NOR-activity during spermatogenesis is common for all vertebrates. However, in human spermatogenesis the absolute sizes of the NORs in the pre- and postmeiotic stages as well as the interphase arrangement of the postmeiotically reactivated NORs show characteristic properties not found in any other species. The cytochemistry of the argentophilic material, the significance of the synthesis of ribosomal RNA in spermatogenesis, and the characteristic arrangement of the Ag-stained NORs in the spermatids are discussed.