Aneuploidy represents the predominant type of chromosomal abnormality found in human newborns with birth defects. The concern that environmental agents may cause aneuploidy in germ cells has prompted development of assay systems for detection of potentially aneuploidy-producing agents. One of the most frequently used methods involves cytogenetic analysis of murine spermatogenic cells at the stages of meiotic metaphases. However, criteria for aneuploidy induction have not been standardized in this test system. Many investigators consider the ability of an agent to induce univalents an appropriate measure of its potential to induce aneuploidy. In the present study, the relationship between univalency and aneuploidy was examined in mouse spermatocytes after various mutagen treatments. 45 Swiss mice were treated with 4 different agents; viz., adriamycin, vinblastine sulfate, cytosine arabinoside, and radiation (cobalt 60) and 10 untreated animals served as controls. From each animal, 50-200 MIs were examined for both sex-chromosomal and autosomal univalency (X-Y U and AU), and equal numbers of MIIs were examined for aneuploidy (hyperhaploidy). No significant correlations between univalency (either X-Y U or AU) and aneuploidy were found in the mutagen-treated mice; nor were they found in the untreated animals. These results indicate that induction of univalents by a mutagen is not necessarily predictive of the aneuploidy-inducing ability of this agent.