A systematic search for chromosome pairing defects in foetal mouse oocytes has been carried out in two different strains (Swiss and CBA/Ca) over days 15-19 of gestation and on day 1 post-partum. The aim was to seek direct cytological evidence for a "production line" of oocyte development, or the occurrence of pairing anomalies at meiotic prophase that might lead, in the adult female, to nondisjunction at anaphase I. No evidence for either was found. The data argue against the "production line" hypothesis as the basis for maternal age-related increases in aneuploidy in the mouse. Attempts to analyse chiasmata in oocytes at diplotene were unsuccessful.