Reproductive biology of Delomys dorsalis (Hensel, 1872)—Rodentia, Sigmodontinae—in an area of mixed forest with conifers, at Rio Grande do Sul, Brasil.Delomys dorsalis is restricted to the wet tropical and subtropical forests of southern and southeastern Brazil and northeastern Argentina. The reproductive cycle of this species was studied by the histological examination of the reproductive tracts of animals caught in an area of mixed forest with conifers (29° 23′ S, 50° 23′ W), in southern Brazil, and from observations on a laboratory group of individuals. Fourteen expeditions were carried out from February 1997 to April 1998, during approximately 3 nights, using 140 live traps set on the ground and on branches between 1.5 and 2.0 m high. Reproductively active individuals were observed year round, although winter was marked by a reduction in breeding activity resulting from recruitment. Females showed a post-partum oestrus and a gestation time between 21 and 22 days. The litter size in captivity ranged from two to four, though pregnant females collected from the field had even five embryos. External reproductive features, frequently used as indicatives of the reproductive status of small mammals in ecological studies, were not accurate and underestimated the number of active animals in the population. Some individuals showed a remarkable scansorial ability that should be considered in future studies about population dynamics.