Abstract During genetic bidirectional selection on mating behavior in Japanese quail (Mahn, 1968) the heart rate was measured routinely in the selected males to ascertain whether a change in the behavior would affect heart rate. The males were injected with a commercial preparation of sodium pentobarbital (Diabutal, Diamond Laboratories, Inc., Des Moines, Iowa), containing 60 mg. of the active ingredient per milliliter. The amount injected subcutaneously in the neck region averaged about 9 mg./100 gm. body weight. Oxygen consumption was determined first and subsequently the heart rate was measured. The bird was placed on its back on a table and held in place by two rubber bands attached to the legs and wings and two nails in the table surface. The heart rate was taken through two electrodes (24 gauge hypodermic needles) which were inserted under the skin close to the ribs on each side of the thorax. The electrodes were…