The intra-atrial electrograms (P waves) from floating orthogonal atrial electrodes of acutely implanted pacemaker leads (SRT lead) were recorded and the frequency characteristics were determined. The atrial sensing properties of the rate responsive pacemaker (RS4) used in conjunction with these leads, were studied in relation to the frequency spectra of atrial electrograms. Whereas the P waves showed a bandwidth to 65 Hz, the filter had an upper cutoff frequency of 35 Hz. We conclude that unreliable atrial sensing with the RS4-SRT pacing system is primarily due to an inappropriate filter match and therefore no satisfactory rate responsiveness is achieved.