A wealth of genomic and proteomic information now makes it possible to analyze fundamental properties and evolution of protein interaction networks. To estimate evolutionary rates, Fraser et al. compared putative orthologous proteins in Saccharomyces cerevisiae and Caenorhabditis elegans. As expected, given that highly interactive proteins are more likely than their less-connected counterparts to be required for viability, highly connected proteins also appear to evolve more slowly. However, the authors' analysis indicates that what limits evolutionary rates of interacting proteins is that a greater proportion of the molecules themselves takes part in physical interactions. Changes in interacting partners are expected to be maintained only if a reciprocal change in the other partner also occurs. This requirement appears to require similar evolutionary rates for the interacting partners.