Loss of normal pacemaker stimulation and/or sensing functions requires prompt detection, automatic correction, and automatic and continuous "marking" of the intermittent failure. The autodiagnostic pacemaker (ADP) detects "failure to capture" (FC) by distinguishing, at its single stimulating and sensing electrode, between the normal biphasic cardiac response evoked by an adequate stimulus (corresponding to the QRS and T waves on the surface cardiogram) and the monophasic pseudo-response generated by electrotonic spread of a subthreshold stimulating current. Detection of "failure to sense" (FS) spontaneous cardiac activity requires two amplifiers: a "timing control" amplifier of standard fidelity and standard (approximately 250 ms) refractory period, and a second amplifier which has negligible refractoriness and provides high fidelity amplification of all evoked and spontaneous activity. Failure to sense (FS) is defined as a specified number of consecutive failures to recycle correctly the pacemaker's timing circuits. Similarly, a specified number of consecutive failures of the stimulus to evoke an active cardiac response is defined as a failure to capture (FC). When FC is detected, the ADP doubles the applied stimulus voltage and generates marker pulses which follow every subsequent stimulus by 40 ms. The marker pulses appear on the surface electrocardiogram, serving as an externally detectable "memory" of the earlier, possible corrected, failure. When FS is detected, non-stimulating marker pulses, of a different time relation (80 ms delay) to each stimulus, are generated continually and can also be detected externally. The ADP has been tested in 14 anesthetized, open-chest dogs. Unipolar rather than bipolar electrodes were used as they rpovided more reliable stimulation and more satisfactory electrograms for detection.