The resting and action potentials and the strength-duration relation to square current pulse stimulation were investigated on the chick embryo''s heart from the 2-day stage (7-somite stage) to near hatching. From the embryos older than 3-days the resting and action potentials were recorded, and the magnitude of both potentials increased with the age. Before commencement of spontaneous contraction (9-somite stage) electric stimulation failed to elicit contraction. After the 10-somite stage all-or-none contraction was produced by electric stimulation, and the strength-duration curves were constructed at all stages by passing cathodal current to the cardiac muscle from 50[mu] micropipette. The rheobasic current was several tens of [mu]A at the earlier stages, decreasing rapidly for the 1st several days, and then falling gradually to less than 1 [mu]A at the later stages. The chronaxie was also found to shorten gradually from about 5 to 1 msec during development. Since chronaxie is proportional to the membrane time constant, this characteristic change in chronaxie (membrane time constant) and rheobase (inverse of excitability) was suggested to correlate to structural change of the cell membrane.