1. Experiments have been performed with adrenaline at dilutions between one part in one billion, 10−12, and one part in ten thousand, 10−4.2. At all dilutions the drug is found to be an augmentor.3. At low dilutions, 10−4 to 10−6, the drug exerts also a different and independent depressing action.4. A physiological method for estimating the rate of disappearance.(oxidation) of adrenaline in Ringer's solution is given.5. A particular dilution of adrenaline, one part in ten millions, is shown(a) to revive an exhausted heart,(á) to make an exhausted heart worse,(b) to accelerate,(b) not to accelerate,(c) to produce tone,(ć) not to produce tone, according to the experimental conditions.6. Hearts do not accelerate with adrenaline unless ventricular conditions make acceleration necessary, extra effort being met primarily by a lengthened stroke of the ventricular pump; acceleration coming in when a lengthened stroke no longer suffices. This result seems likely and reasonable, but it implies that the ventricle can influence the pacemaker's rate.7. Local antidromic impulses are normal factors in cardiac coordination.