Abstract: In this research, we presented on each trial three cards with 2 to 6 black-filled circles to monkeys. The monkeys were required to sum two of the three numbers and to determine which one(the sum or the third one) had the greater number of dots. The monkeys were trained on nine tasks but only one monkey met the criterion of all tasks. We concluded that monkeys were able to learn to sum up two numbers together, Our explanation is that the monkey's performance is due to perceptual fusion.

Key words: rhesus monkey, summation, number descrimination, perception fusion