We used a prospective seroepidemiological study, in conjunction with a mark-release-recapture protocol, to investigate the transmission of hantaviruses in four rodent species from Baltimore, Maryland, from June 1984 to June 1986. A total of 1,208 captures of 762 rodents provided 984 individual blood samples. The antibody prevalence, as determined by frequency of reciprocal indirect fluorescent antibody (IFA) titers ≥32, was 33.9% in rats (Rattus norvegicus, n = 466), 28.3% in meadow voles (Microtus pennsylvanicus, n = 67), 1.4% in house mice (Mus musculus, n = 146), and 1.2% in white-footed mice (Peromyscus leucopus, n = 83). Populations of all rodents were maximal during the fall and winter months, but population trends were not clearly associated with periods of virus transmission. The mean incidence of seroconversion to a Hantavirus for rats was 12.06/100 rats/month, but incidence rates could not be established for other species. Rats which seroconverted were generally sexually mature animals, and there was evidence of transmission throughout the year. Animals which seroconverted to a Hantavirus achieved high IFA titers, and remained seropositive for the duration of the study.