Summary: In telecommunication systems one of the basic elements is a multiplexer, which connects a number of input lines via a buffer to an output line. We consider a fluid model of a multiplexer loaded by a finite number of heterogeneous on-off sources. Under certain assumptions, the output from the multiplexer looks like another on-off source. Our main result is a generalisation of a result by H. Kaspi and M. Rubinovitch [Stochastic Processes Appl. 3, 73-88 (1975; Zbl 0321.60070)] characterizing the active periods on the output line by means of Laplace transforms.