This paper has two main purposes: first, it studies the impact of election closeness on political information demand and supply; second, it provides evidence on mass media behaviour during electoral campaigns. The theoretical part presents a model of information demand and supply. If voters act instrumentally then information demand should increase with marginality. Mass media are modeled as profit-maximizing firms that take into account information demand, the value of customers to advertisers and the marginal cost of customers. Information supply should be larger in constituencies where the contest is expected to be closer, there is an higher population density, and population is on average more valuable to advertisers (for example richer, better educated etc.). The impact of electorate size is theoretically undetermined. These conclusions are then tested with comfortable results on data for the 1997 general election in Britain.