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1. Firing patterns of single atrial fibres were monitored in order to study their sensitivity to rate of change of atrial volume and their dependence on receptor location in the atria.

2. To that effect, sinusoidal fluctuations of volume (±1 ml.; 60/min) were superimposed on the natural pulsations of the heart of spontaneously breathing anaesthetized cats.

3. Stimulus—response histograms revealed that maximal firing frequency of B-impulses preceded maximal volume change by 10-80° (average 48±7° (S.E.), thirteen right and left atrial receptors) indicating that sensitivity to rate of change of atrial volume at least equalled volume sensitivity.

4. The rate sensitive component of B-firing increased with respect to the volume sensitive component when atrial filling diminished and was larger for high threshold receptors than for low threshold receptors (P<0·001).

5. The A-bursts were not affected by atrial pulsing in eight out of nine anatomically localized receptors; inspiration prolonged the A-bursts of three receptors and advanced the timing of A-bursts with respect to the a-wave (10-50 msec) of all nine receptors.

6. About half of all anatomically localized receptors were traced to the interatrial septum. These septal receptors differed from right and left atrial receptors as follows: (a) they were exclusively of the AB-type (fired during atrial a- and v-wave); (b) their B-firing rates were significantly lower (P<0·05); (c) A-firing patterns remained largely unaffected by respiration or by atrial pulsing; (d) they signalled volume changes of either atria; (e) no association of rate sensitivity with threshold was detected (P>0·95). The physiological role of septal receptors remains to be elucidated.

7. It was concluded that B-fibres were sensitive to both change and rate of change of atrial volume; their role as sensors of venous return is suggested strongly. Results further suggested that A- and B-firing are determined largely by the location of atrial stretch receptors.