A reverse transcriptase-polymerase chain reaction assay (RT-PCR) was used quantitatively to measure accumulated levels of RNA transcripts in total mouse RNAs derived from male germ cells at various spermatogenic stages. RNA levels for two X-linked enzymes, phosphoglycerate kinase (PGK-1) and hypoxanthine phosphoribosyl transferase (HPRT), both decrease during spermatogenesis, although the transcript levels decrease much more rapidly for PGK-1. RNA for the Y-linked ZFY (zinc finger protein) is elevated in all spermatogenic cell fractions tested, being particularly high in leptotene/zygotene spermatocytes and round spermatids. RNA for adenine phosphoribosyltransferase (APRT) increases 5-fold to a peak during late pachynema. RNA for PGK-2, undetectable in spermatogonial cells, increases at least 50-fold by the round spermatid stage. DNA (cytosine-5-)-methyltransferase (MTase) transcript levels are over an order of magnitude higher throughout spermatogenesis than in non-dividing liver cells.