Fertilized Leghorn eggs were incubated at temperatures between 37.6 and 38.2 degrees C at a relative humidity of 60%. Electrical, mechanical and optical techniques were tested to find a non-invasive method for recording the embryonic cardiac activity. By means of electrical recording techniques no appropriate signals could be picked up with the shell being intact. Difficulties arose from potential changes generated by the system shell-shell membranes in contract with the electrolyte solutions. When the shell was locally removed, ECG-analogical potential changes could be recorded in these circumscribed areas from the 14th day of incubation, provided that conditions favoured the projection of the integral vector on the direction of the leads. Recording conditions were considerably improved when the holes in the shell were located above large vessels. Under such conditions, the signals of cardiac activity were mainly produced by pulsation of the vessels at the area of contact of the electrodes. However no improvement of the results could be obtained by chronical implantation of the electrodes; this method frequently impaired the course of incubation. Attempts to record cardiac activity by means of the mechanical systolic impulse failed because of the practical impossibility of reducing external vibrations below the level of the signal. The best reproducible results, being also the least susceptible to disturbances, were attained by measuring the variations of the current of the light flowing through the egg. By this method, the heart rate could reliably be recorded already from the 3rd day of incubation without any impairment of the embryo. The reliability of the optical method was checked by different modes of application.