RESUMEN
Heterozygous and homozygous nude female mice with the genetic background BALB/c were housed in a clean conventional colony at the Laboratory Animal Facilities of Instituto Butantan. Nude animals are known to have severe deficiencies in reproductive function. Nude female is usually sterile where only 8.8 percent of them present spontaneous fertility. In order to study the involvement of the thymus for these reproductive defects, athymic nude female at different stages of development were treated with thymocytes suspensions and mated with heterozygous males. The results obtained for nude females at 10 and 30 days of age were respectively: a) number of pregnant = 48 percent and 12 percent; b) number offspring per female = 5.0 and 4.0; c) natality rates were 2.40 and 0.48; and d) under conditions used in this study the life span for nude animals was prolonged until 7 months. The use of this treatment would increase the animal numbers, which are important research models for biomedical investigations. Considering that nude mice are difficult to raise, an obtainment of these animals in higher number by this method appear to be an essential event, and might offer to grow better for the animals, which are used for breeding