The interfacial electric-field interaction subdivides into four cases: the HH, VV, HV, and VH cases. For each case, we derive the impulse-response solution of the interfacial electric field generated by an infinitesimal impulse current source at the same interface, using plane-time Fourier transform and a variant scheme of the Cagniard-de-Hoop method. We show that it is possible to obtain exact explicit-form solutions for the HH and VV cases, whereas we have to evaluate the electric fields only by numerical calculations for the HV and VH cases since the exact solutions are yet unavailable. Various aspects of the impulse-response solutions, such as the impulse radiation characteristics and the transient responses to time-Gaussian inputs, are investigated as well as the offset-differentiated field characteristics for the HV and VH cases. We also discuss the effect of the lower-medium loss on the transient waveforms and the effect of the interface that makes the interfacial interaction quite different from the interaction in free space. ¿ 1999 American Geophysical Union