The solution of relational size problems in the context of ordinal series was facilitated when children were directed to locate a terminal member (the smallest) of the series. In contrast, subjects who were instructed to locate a nonterminal member without prior designation of a terminal member were unable to achieve solution of the problem. Age-and sex-related differences in performance were observed as well as differences associated with stimulus arrangements and stimulus sizes. The outcomes were discussed in terms of composite task demands.