The identification and progression of the prophase stages of meiosis in the mouse foetal ovary are reported, from d 13 of gestation to d 1 postpartum. Air-dried Giemsa-stained oocyte preparations are compared with surface-spread silver-stained cells. The latter method allows a more detailed quantitative analysis of the pachytene stage. Numbers of synaptonemal complexes can be counted, and the degree of synapsis determined. The progression of cells appears to be relatively synchronous, in agreement with previous reports. The activity of nucleolar organisers, in particular one associated with the shortest synaptonemal complex (chromosome No. 19) is described. At late pachytene the lateral elements of the No. 19 bivalent desynapse precociously with apparent nucleolar involvement.