For certain reasons I wished to train a young chimpanzee to choose from two similar boxes the one characterised by the ticking of a metronome inside it. My subject was a young male chimpanzee (Pan leucoprymnus Lesson), approximately three years old, Tommy by name. He was a good-natured chap, quite tame, and already for some years in captivity. When we knew each other a little better, he would welcome me every morning with a hearty “uhuh”, as soon as he heard my steps, and after the experiments of the day we would take leave by shaking hands as old friends. Tommy was rather lazy and not so playful as most young chimpanzees, but this quality made working with him easier and more regular. With a few exceptions it did not give much trouble to make 50—60 trials with him each day.