There are several reports demonstrating that aneugens may preferentially affect segregation of particular chromosomes in somatic cells. Much less is known on specific susceptibility of individual chromosomes to non-disjunction in mammalian meiosis in response to chemical exposures. To explore possible chromosome-specific behaviour and susceptibility to errors in chromosome segregation in mammalian oogenesis we employed spindle immunofluoresecence in combination with FISH with chromosome-specific probes to analyse congression of chromosomes X, 8 and 16 in diazepam (DZ)-treated, meiotically delayed meiosis I oocytes of the mouse. Concomitantly, we assessed the susceptibility of homologues to precociously segregate prior to anaphase I during DZ-induced meiotic arrest. About 50% of all oocytes exposed to 25 microg/ml DZ became meiotically delayed. Chromosomes failed to congress at the spindle equator in one-third of these meiosis I oocytes. The X chromosome was significantly more often located away from the spindle equator as compared with the expected random behaviour. Concomitantly, DZ exposure induced untimely segregation of homologous chromosomes of the gonosome and the autosomes in meiosis I. This occurred with similar frequencies. The observations confirm that DZ perturbs cell cycle progression, interferes with chromosome alignment, causes predivision and thus may predispose mammalian oocytes to errors in chromosome segregation. For the first time, chromosome-specific behaviour is reported in female meiosis in response to exposure to an aneugenic chemical.