The present invention discloses a method for carrying out a loopback test in a data communication system having a first data communication station, a second data communication station and a transmission line connected therebetween. The first data communication station transmits a succession of a first pseudo-random noise signal, a loopback test signal and a second pseudo-random noise signal to the second data communication station by utilizing a usual information data channel, and requires no special channel for carrying out the loopback test. When the second data communication station detects the first pseudo-random noise signal, it transmits the loopback test signal to the first data communication station in order to check the coincidence of the transmitted and the received loopback test signals. The second pseudo-random noise signal denotes the end of the loopback test.