Captive-bred Mus musculus (house mice) and Apodemus sylvaticus (field mice) were each infected with 50 oocysts of Toxoplasma gondii M1 strain per os and infection in them and their offspring was assessed by polymerase chain reaction (PCR) amplification of the T. gondii B1 gene in brain tissue and by serology, using the modified agglutination test (MAT). The chronically infected female A. sylvaticus (n=10) and M. musculus (n=23) were mated at least 6 weeks after infection (and subsequently to produce up to 6 litters) and their pups examined 3 weeks after weaning at 6 weeks of age. By PCR, in offspring of A. sylvaticus and M. musculus respectively, vertical transmission was demonstrated in 82·7% (n=83) and 85·0% (n=207) of all pups (N.S.,P>0·05), 95% (n=21) and 100% (n=30) of all litters(N.S., P>0·05), with a mean (±S.E.) proportion of each litter infected of 0·87 (0·06) and 0·86 (0·04) (N.S., P>0·05). There was no change in any of these variables between first and subsequent litters. By serology, whilst MAT suggested 100% vertical transmission in A. sylvaticus, it under-estimated rates of infection in offspring of M. musculus. A limited series of bioassays from M. musculus tissues confirmed the good correlation of PCR and the poor correlation of MAT with mouse inoculation. These results indicate that vertical transmission in A. sylvaticus and M. musculus is extremely efficient and probably endures for the life of the breeding female. This mechanism favours parasite transmission and dispersion by providing a potential reservoir of infection in hosts predated by the cat.