Unipolar and bipolar floating atrial electrograms from 58 pacemaker patients were recorded and compared. Twenty-four floating unipolar electrodes and 29 floating bipolar electrodes were used at mid-right atrial level and five orthogonal atrial J leads within the right atrial appendage. Each signal was analyzed in the time domain: peak to peak deflection of P wave and QRS complex, duration of P wave and QRS complex and slew rate; and in the frequency domain: maximum of the energy spectrum and frequency at which a decrease of 3 dB from the maximal amplitude occurred. Atrial P (1.31 +/- 0.94 mV, mean +/- SD) and QRS (1.0 +/- 0.56 mV) waves from unipolar floating electrodes were comparable, whereas they were significantly different from bipolar floating electrodes (1.15 +/- 0.77 mV and 0.25 +/- 0.39 mV). Amplitudes of P waves from orthogonal J leads were largest (3.1 +/- 2.6 mV) and QRS complexes (0.21 +/- 0.13 mV) smallest. The P waves had the highest frequency content (17.1 +/- 19.4 Hz). It is concluded that atrial electrograms from orthogonal electrodes (bipolar or orthogonal J) offer superior sensing characteristics because of the large amplitude P wave and discriminating power between P and QRS waves (P/QRS voltage 15:1). An orthogonal J lead can thus be used for P synchronous pacing at the atrial level, whereas an orthogonal ventricular lead can be used for rate-response pacing systems.