The guinea pig and its relatives are promising candidates as animal models for studying trophoblast invasion. The origin, migration routes and kinetics of invasive trophoblast cells were examined in two caviomorph species. Histology and immunohistochemistry were done on placentas from 38 guinea pigs of days 20-47 and 13 degus of days 25-51 of gestation. BrdU was used as an in vivo marker for proliferation and for tracing of migration routes in the placenta; it was injected 24h to 15 days before collecting the material. In both species extravillous-like trophoblast cells are derived from proliferating stem cell aggregations in the subplacenta, which are comparable to the cell columns in humans. Migration routes and kinetics under in vivo conditions revealed a mean invasive depth of 300-350 microm/day and a mean life span of the extravillous-like trophoblast of 30 days. The patterns of trophoblast invasion in caviomorphs are analogous to the situation in humans, suggesting that these rodents are appropriate animal models for the study of the dynamics of trophoblast invasion.