The responses of two social groups of Cebus apella to novel or familiar objects were examined, both in scarce and in abundant conditions. The aim was to obtain a description of capuchins' general norms of behavior toward objects, focusing on the variability across sex/age classes. Plain wooden blocks were presented to each group of monkeys in four phases. In phase I, the blocks were scarce and novel; in the following phases the blocks were abundant and increasingly more familiar. Several categories of behaviors directed toward the blocks were scored. The blocks elicited high levels of responsiveness throughout the experiment. In both sexes, responsiveness tended to decrease across phases. In all phases, males interacted with the blocks more than did females. Age was a significant determinant of responsiveness. Furthermore, age-classes showed different trends in level of responsiveness across phases. Sex and age significantly affected the behavioral pattern of interaction with the blocks. Dominance did not seem to constrain monkeys' responsiveness. The high and sustained responsiveness toward objects by Cebus apella is consistent with their manipulative skills and their varied habitat exploitation.