Population ecology of small mammals was examined over a 14-month period in the tropical gallery forests of the cerrado of central Brazil. Mark-recapture techniques provided detailed information on age structure, population dynamics, biomass, and patterns of reproduction for six species of oryzomyine rodents (Oecomys bicolor, Oecomys concolor, Oryzomys capito, Oligoryzomys eliurus, Rhipidomys mastacalis, and Nectomys squamipes) and two species of marsupials (Didelphis albiventris and Gracilinanus agilis). Different species show distinctive patterns of population increase and reproductive activity, with some maintaining stable populations throughout the year and others having a boom-and-bust pattern. Those that show pronounced differences in population size at different times of the year generally reach their lowest population levels during the region's pronounced dry season. Among the smaller-bodied species, the great majority of mammalian biomass on the study area is contained in the rodents. If the larger-bodied species are included, however, the marsupials comprise most of the biomass. Most species did not appear to be long-lived. Only D. albiventris persisted on the grid for >12 months; the majority of species lasted <6 months. The tropical gallery forests of central Brazil support a small-mammalian fauna that is at least as rich and complex as any other tropical site that has been examined to date and may support higher densities than any other site heretofore examined.