Autoradiographic and electron microscope methods were used to correlate changes in nucleoproteins with nuclear fine structure during spermatogenesis in the mouse. Testes were fixed at daily intervals after intratesticular injectionwith labeled amino acid. [3H]Arginine, lysine, valine, and proline were rapidly incorporated into primary spermatocyte nuclei, retained through subsequent spermatocyte divisions and through spermatid differentiation to step 12 of spermiogenesis, but were lost with spermatid differentiation beyond step 12. Arginine and lysine (not valine or proline) also were rapidly incorporated into certain elongated spermatid nuclei but differed strikingly in their distribution and fate. Nuclei of late step-12 through step-15 spermatids were initially labeled with arginine. This label was retained through subsequent spermatid differentiation and sperm maturation in the epididymis. By contrast, lysine was initially incorporated only into late step-12 and step-13 spermatid nuclei, and was retained only to early step 14 of spermiogenesis. Spermatid incorporation of lysine coincided with the initiation of chromatin condensation in late step-12 nuclei, and loss of lysine coincided with the completion of condensation in step-14 nuclei.