The analysis of the fertility of hybrid and nonhybrid individuals from a chromosomal contact zone gives us the possibility of studying the role of chromosomes in speciation processes. In this study, homozygous, "simple" and "complex" Robertsonian heterozygous male common shrews (Sorex araneus) from the Abisko-Sidensjö chromosomal hybrid zone in Sweden were analysed. The degree of germ cell death was estimated, sperm counts were performed and the testis and seminal vesicles weighed in each individual. Chromosome interactions and synapsis at pachytene were examined under the EM. The weight of the testis was significantly different in the three karyotypic groups and "complex" heterozygotes suffered higher germ cell death than homozygotes and "simple" heterozygotes. Interactions between chromosomes at pachytene were rare. Nonhomologous pairing in the centromeric regions of autosomal trivalents (side arms) was common while asynapsed segments were seldom found. Thus, the difference in the reproductive characteristics of the common shrew might be due to genetic factors rather than Robertsonian translocations.