Background. Findings from previous research into motivation in young children with Down's syndrome (DS) have been mixed. Some studies have suggested that development is merely delayed, while others have proposed that there are inherent differences or deficits. Using the mastery motivation paradigm, studies of young children have often found that those with DS are just as persistent and goal-directed as typically developing children of the same mental age (MA). However, research involving children with DS with MAs above 2 years is very limited. The major aim of the present study was to extend previous research by focusing on children with MAs between 24 and 36 months. A secondary aim was to investigate issues which would advance conceptual knowledge about the construct of mastery motivation. Method. The participants were 25 children with DS and 43 typically developing children, matched for MA (24-36 months). The main measures of mastery motivation were persistence with structured mastery tasks (i.e. puzzles and shape-sorters) and maternal reports. Results. With the challenging tasks, children with DS were just as persistent as the typically developing children. Correlations of persistence measures in the group with DS suggested that persistence for these children represented a more generalized approach rather than a task-specific response. Maternal ratings of persistence were lower in the group with DS. Conclusions. The main conclusion was that children with DS in the MA range of 24-36 months do not differ in their persistence with challenging tasks when compared with typically developing children of the same MA. The implication is that motivational development is delayed for children with DS, rather than deficient. However, there were some indications of possible differences in the processes underlying mastery behaviour in the two groups. The study addresses a number of conceptual and methodological issues associated with mastery motivation research, and stresses the important contribution that future longitudinal studies could make.