The cognitive determinants of number analogy performance were studied by systematically manipulating the processing demands imposed by the items. To explore sources of developmental differences in analogical reasoning, subjects were included from two age levels, grades 4 and 5 and college. To allow the investigation of individual differences in reasoning ability, the children were selected to represent two general aptitude levels, average IQ and high IQ. The experimental number analogies correlated highly with the number analogies portion of a standardized intelligence test: the Analysis of Learning Potential. In each group a combination of domain-specific and general procedural knowledge factors accounted for more than 70% of the variance in item solution difficulty. The most critical processing demand affecting successful performance was the amount of solution-related information to be assembled and managed in working memory. Domain-specific and general procedural knowledge factors differentiated adults from children, while only specific, factual knowledge competencies seemed to differentiate between IQ levels in children. (Author/CP)