SUMMARY1. The effect of increases in heart rate on plasma atrial natriuretic peptide (ANP) concentrations was investigated in conscious rabbits.2. Plasma ANP concentrations were significantly increased following abrupt increases in heart rate produced by atrial pacing at 400 beats/min. Pacing at 300 beats/min resulted in smaller increases in plasma ANP concentration.3. Stepwise increases in heart rate produced by atrial pacing at 250, 300, 350 and 400 beats/min resulted in increases in plasma ANP concentrations at 400 beats/min only.4. The increase in plasma ANP concentration during atrial pacing correlated significantly with the increase in heart rate achieved.