Abstract: The purpose of this study was to apply Piaget's theory and methods to in- vestigate the cognitive abilities of rhesus monkeys and assamese macaque. Do monkeys have prerequisite skills for conservation of quantity? Are there any evidences for conservation of quantity in monkeys? And specifically, might they respond differentially to equivalent and unequivalent numerousness cues? Four monkeys were trained to make sameness-difference judgments of numerous- ness. Several controls were used to insure that the monkeys did not merely learn specific stimulus properties or patterns of stimuli. Three monkeys responded significantly and differentially to pairs of same numerousness versus pairs of di- fferent numerousness. They met criterion in 80, 160, and 200 trials respective- ly on the final task. The discussion includes problems of Piaget's theory and method involving conservation of quantity in primates and possible underlying processes to explain the numerousness judgments.

Key words: Primate, Piaget's theory, Conservation of quantity, Sameness-difference judgment