Summary: \textit{N. Meggido} [Math. Program. 7, 97-107 (1974; Zbl 0296.90048)] showed that the maximum flow through sets of sources in a multiple sink flow network is a polymatroidal function. Recently, \textit{A. Federgruen} and \textit{H. Groenevelt} [``Polymatroidal network flow models with multiple sinks: Transformation to standard network models'', Res. Rep., Grad. School Business, Columbia Univ. 1985; Manage. Sci. 32, 341-349 (1986; Zbl 0603.90071)] have considered polymatroids that can be represented by a multiple source flow network and have improved the running time of an important scheduling application. We give a characterization of network representability and relate representable polymatroids to the theory of grammoids.