Chiamata were counted and paired and unpaired configurations at first meiotic division and chromosome errors at second meiotic division were assessed at different ages in males and females of two strains of laboratory mice. In the females a decrease of chiasma frequency and an increase of univalents at first meiotic metaphase (MI) were confirmed. In the males, diakineses had higher chiasma frequencies (in the range of the female MIs) and fewer univalents than the MIs had. In these male cells there was no decrease of chiasmata or increase of autosomal univalents with age, and there were some interstrain differences. In the older females there was no parallelism between the frequencies of univalents at MI and the chromosome errors that could be identified at second meiotic division; these were fewer than might be expected on the assumption that all the univalents were true univalents. The relevance of this finding to the question of the nature of most of the univalents observed at first meiotic division in aged female mice is discussed.