_P. dimidiatus_ was studied in the field and laboratory. _P. dimidiatus_ in captivity showed regular diurnal activity, coming out of the nest for food at intervals. Individuals were fed a cereal mixture and nuts of small size. The animals usually buried the nuts in the sand of the outer cage. While holding the nut with the mouth and front feet, the animal patted the sand rapidly, thus burying the nut, and it then pushed more sand over the place with the front feet. Sometimes the emergence from the nest is followed by a long yawning and stretching ceremony. The animal spreads the fore and hind legs widely apart, while the back is curved down and the head and tail turn upward. Then one of the hind legs is stretched backward and, at the same time, the mouth is opened widely and the tail is moved in an undulatory fashion. The operation may be repeated using the other legs, or not. _P. dimidiatus_ was regularly found in climax forest. The best shelter and nesting ground was usually under boulders, commonly not farther than 10 meters from water. The entrance to the nest was kept clean. No more than two adult animals (male and female) were captured in the same shelter, and only a few times were young captured in the same place with adults. Nesting places were located also at the bases of trees and near fallen logs where litter accumulates. Records of animals kept in captivity show that the species _dimidiatus_ survives more than two years. Specimen MN no. 5448 [M] was adult when captured by the Serviços de Estudos e Pesquisas sobre a Febre Amarela on December 5, 1938, and died on January 17, 1942. Therefore, it lived for more than 1,139 days. _Proechimys dimidiatus_, in Rio de Janeiro, as well as _P. i. bonafidei_ and _P. i. iheringi_ which live in the same region, were found breeding from September to November and from March to May. _Proechimys longicaudatus roberti_, in Anapolis, Goiaz, was found breeding from July to November and from January to March. _P. g. hyleae_ and _P. g. leioprimna_ in the lower Tapajoz and lower Tocantins rivers, Pará, were found breeding in January. It seems that in the Central Plateau and southeastern Brazil, _Proechimys_ has two litters per year, one in the early spring and a second in the late summer. The number of young per litter varies from 1 to 5, although 2 is the usual number. CHANGES WITH AGE _Juveniles._--The animals are born with a thick pelage of thin aristiforms and thin setiforms. The color is uniformly blackish brown. The nose, hands, feet, ears and tail are pinkish; P4 and M1 are already erupted and the second molars are included in the bony alveoli. The incisors are orthodont; the rostrum is short and the braincase is wide. The posterior part of the skull is greatly curved dorsally. No change is noticed in the pelage before the second molars erupt and become functional.