Autoradiographic procedures for the study of RNA and protein synthesis during spermatogenesis have been complemented with electron microscope techniques for visualization of gene activity. These procedures have enabled us to determine that RNA transcription is highly selective with respect to RNA species, timing of synthesis, types of chromosomes (autosomes and sex chromosomes), segments of chromosomes (i.e., the lampbrush segment), and chromatin structure. In mouse and human spermatocytes, a peak production of ribosomal RNA (rRNA) occurs during leptotene-zygotene, preceding nonnucleolar RNA synthesis, which is at a peak in middle pachytene. Transcription in late spermatids decreases in coincidence with changes in chromatin structure and high incorporation rates of [3H]arginine. In these cells, a particulate repeating pattern of chromatin is replaced by chromatin fibers of uniform diameter as highly arginine-rich proteins replace somatic histones. In spermatogonia, spermatocytes and Sertoli cells, the products of transcription are mainly heterogeneous nuclear RNA (hnRNA) and rRNA, whereas spermatids transcribe predominantly hnRNA during early spermiogenesis. Persistent long-lived [3H]-uridine-labeled RNA species in pachytene spermatocyte nuclei contrast with a fast turnover of [3H]uridine-labeled RNA in Sertoli cells as detected at the same pulse labeling time (8--12 days). From these results one can postulate a still undefined control mechanism of gene expression during spermatogenesis for modulating a cascade of events required for male gamete formation.