1. We recorded the electrical activity of single afferent cardiac fibres isolated from the third and fourth left thoracic sympathetic rami communicantes of anaesthetized cats. Their conduction velocities ranged from 12 to 32 m/sec.2. The endings of each fibre were localized to one cardiac chamber by mechanical probing of the opened heart performed at the end of the experiment.3. The impulse activity was spontaneous and, in fibres with atrial or ventricular endings, it was in phase with a particular atrial or ventricular event.4. This nervous activity increased during increases in pressure occurring in the chamber where the endings were located. Conversely, decreases in pressure were accompanied by decreased nervous discharge.5. In some experiments the left coronary artery was perfused at different flows and pressures. Brief decreases or increases in coronary flow and pressure decreased or increased, respectively, the discharge of fibres with atrial or ventricular endings. Fibres were excited by intracoronary injections of veratridine.6. Cessation of coronary pump flow increased the discharge of fibres with atrial or ventricular endings only when myocardial ischaemia was accompanied by signs of heart failure.7. These afferent cardiac sympathetic fibres which provide the spinal cord with continuous specific information on cardiac events are likely to contribute to the neural control of circulation.