Nativist and constructivist accounts differ in their characterization of children's knowledge of grammatical categories. In this paper we present research on the process of acquisition of a particular grammatical system, gender agreement in the Spanish noun phrase, in children under three years of age. The design of the longitudinal study employed presents some variations in relation to classical studies. The aim was to obtain a large corpus of NP data which would allow different types of analysis of the children's productions to be carried out. Intra-individual variability in early NP types was analyzed and measured, and an elicitation task for adjectives was used. Results show that the acquisition of NP and gender agreement is a complex process which advances as the children gradually integrate different pieces of evidence: phonological, distributional and functional. The reduction of variability as the grammatical process advances is a key feature for its explanation.