Let G be a graph with a perfect matching M. Define the forcing number of M in G to be the smallest size of a subset S subset of M that is in no other perfect matching. In this paper, we present a property of bipartite graphs G that acts as a lower bound on the forcing number of perfect matchings in G. We then apply this to the torus and the hypercube, proving that the minimum forcing number of a perfect matching on a 2m x 2n torus with m greater than or equal to n is 2n, and that the minimum forcing number on an n-dimensional hypercube is 2(n)/4 if n is even. (C) 2002 Elsevier Science B.V. All rights reserved.