This paper aims at investigating the acquisition of number agreement within DP by a child acquiring Brazilian Portuguese, under the assumptions of the Minimalist Program (Chomsky, 1998; 1999). In the analysis of the child's speech production, I highlight the most frequent patterns of number agreement and plural marking, and discuss three hypotheses (Carstens, 2000; Lopes, 2004; Magalhães, 2004) to find the one which explains the variation within the child's own speech and also between child and adult registers; such hypothesis should also be able to predict how the infant's number agreement patterns will evolve into the adult system. The proposal by Carstens (2000) is the only one which supports the patterns found in the data under analysis, especially the variation between the adult patterns of agreement and plural marking vs. the occurrence, in child language, of plural marking morpheme (-s) only on nouns, which is ungrammatical in adult registers.