Described is a portable, battery-operated electrocardiography (EKG) transmitter, and method of transmission, from the patient's location via telephone lines to a diagnostic central. The transmitter, which includes a 'calibration'-signal generator for producing a reference signal, an EKG-signal amplifier and a frequency-modulator, operates at a nominal carrier frequency of 1800 HZ. Each (periodic or emergency) transmission commences with a sequence of calibrate-signal square waves at about 5 Hz for about 5 seconds. The calibrate signals modulate the 1800 Hz carrier to produce a 225 Hz deviation and deflect the EKG recording pen at the diagnostic central by 1 cm. The diagnostic central maintains a fixed sensitivity of 1 cm deflection per 225 Hz=.DELTA.f as a reference signal for all patients. At the end of the 5 second interval, the calibration signal generation terminates automatically, and then the amplified EKG-signals are switched to modulate the carrier. The reference signal, as pen-recorded preceding and juxtaposed to the recorded EKG-signals proper, serves to provide: (1) a convenient visual reference and an accurate calibration marker for the EKG-signal amplitudes, to indicate changes in EKG signals relative to previous transmissions; (2) a basis for discrimination between changes attributable to physiological changes in the patient and changes attributable to aging and drift of transmitter components; (3) a forecast of failure of a patient's pacemaker; (4) early warning of the approach of end of the battery life. However, long battery and low drift are assured by automatic shut-off of the transmitter after operational use thereof for about 90 seconds maximum, and by selection of stable components.