This article describes part of the findings of a teaching experiment whose objective is to investigate the algebraic abilities of elementary students when they solve situations that involve a functional relationship. In particular, we focus on describing the use and meanings attributed to letters by third-year primary school students when faced with verbal problems related to the generalisation of a functional relationship. Drawing from the functional approach to early algebra and set in Spain, the study expands on earlier research conducted on primary school students' use of letters in algebraic contexts. Their initial reactions to the use of letters to represent indeterminate quantities and how those reactions changed in the course of three sessions are described. Analyses of the students' written answers together with their participation in group discussions yield qualitative data on how students associate the idea of variability with indeterminate quantities and use letters, numbers or both to represent that notion.