This work is concerned with the acquisition of the Portuguese number system. Longitudinal production data of two children (24-28 months and 21-25 months) were collected for 4 months in order to verify whether there is evidence of the use of number inflected forms. 3 picture- identification experiments were carried out with children ranging from 18 to 28 months and 30 to 42 months, whose aim was to verify (i) whether children would distinguish number inflected from number uninflected nouns (ii) whether they would process number agreement in the DP and the extent to which they would perceive number incongruence and ungrammaticality as far as number agreement in the DP is concerned. Both nouns from children`s vocabulary and pseudo- nouns were used. The latter was indented to constraint semantic information to the number affix. These data were contrasted with the results on gender (Name, 2002), which suggest that young children (mean age 24 months) perceive gender incongruence between D and N in the processing of a DP. The working hypothesis of this study is that children identify morphological information concerning number and gender within the closed class of determiners. The fact that number is basically an optional feature, which can be expressed in the Noun, would nevertheless, make the acquisition of grammatical number to be relatively more demanding than the identification of grammatical gender. The linguistic framework assumed here is the Minimalist Program (Chomsky, 1995).