Cellular networks composed of metabolic, signalling, and genetic subnetworks, comprise many distinct intermediates. However, only a subset of the latter, referred to as 'communicating intermediates', mediate cross-talk between individual modules. Here, this characteristic feature of modular networks is exploited to simplify the quantitative description of the responses of these networks to environmental changes, to a description solely in terms of the communicating intermediates. Such a strategy reduces the number of variables that need to be considered. It allows for the determination of the quantitative contribution of individual modular interactions to the regulation of concentrations of communicating intermediates. This is illustrated by a calculation for an example of a modular biochemical network.