Abstract. The preconditions and early steps of meiotic chromosome pairing were studied by fluorescence in situ hybridization (FISH) with chromosome-specific DNA probes to mouse and human testis tissue sections. Premeiotic pairing of homologous chromosomes was not detected in spermatogonia of the two species. FISH with centromere-and telomere-specific DNA probes in combination with immunostaining (IS) of synaptonereal complex (SC) proteins to testis sections of prepuberal mice at days 4-12 post partum was performed to study sequentially the meiotic pairing process. Movements of centromeres and then telomeres to the nuclear envelope, and of telomeres along the nuclear envelope leading to the formation of a chromosomal bouquet were detected during mouse prophase. At the bouquet stage, pairing of a mouse chromosome-8-specific probe was observed. SC-IS and simultaneous telomere FISH revealed that axial element proteins appear as large aggregates in mouse meiocytes when telomeres are attached to the nuclear envelope. Axial element formation initiates during tight telomere clustering and transverse filament-IS indicated the initiation of synapsis during this stage. Comparison of telomere and centromere distribution patterns of mouse and human meiocytes revealed movements of centromeres and then telomeres to the nuclear envelope and subsequent bouquet formation as conserved motifs of the pairing process. Chromosome painting in human spermatogonia revealed compacted, largely mutually exclusive chromosome territories. The territories developed into long, thin threads at the onset of meiotic prophase. Based on these results a unified model of the pairing process is proposed. p AIRING of homologous chromosomes during meiotic prophase of sexually reproducing organisms culminates in the formation of the synaptonemal complex (SC) 1 (for reviews see von Wettstein et al., 1984;Giroux, 1988). The SC is composed of axial elements (cores) that connect sister chromatids along their entire length. These become lateral elements when they get interconnected by transverse filaments to result in the well-known tripartite SC structure (Schmekel and Daneholt 1995). While chromosome pairing and meiotic recombination Address all correspondence to H. Scherthan,