A simple electronic network employing unijunction transistors has been used to obtain qualitative information regarding the entrainment behavior of relaxation oscillators coupled by low pass filters. Using arbitrary criteria, entrainment boundaries have been determined over the frequency range from 0.14 to 0.5 Hz for filters having time constants of 0.55,1.1 and 1.7 s. It is shown that the efficacy of entrainment is related to filter time constant and the Fourier structure of the bilaterally coupled signals