RESUMEN
Two parameters, mammary gland morphology and liters' weekly weight gain were involved in this study, to assess a superimposed pregnancy on lactating female. Thirty five, female, Norway, albino rats [Rattus norvegicus] 7-10 days lactating were mated with males. When a new pregnancy occurred, litters of the pregnant-lactating rats were weekly weight. Two weeks after pregnancy, the pregnant-lactating rats were sacrificed, and their mammary glands histology were studied. Simultaneously, other five lactating and five pregnant rats were sacrificed. These two groups were used as controls. No significant differences were seen in the weekly weight gain between the litters of pregnant-lactating and lactating rats. In addition mammary glands of both groups were histologically identical. Yet, these groups showed a completely different mammary gland morphology, when they were compared with pregnants only. This study showed that, a new pregnancy carries a trivial influence on a well established lactation