As roots explore the soil, they encounter a complex and fluctuating environment in which the different edaphic resources (water and nutrients) are heterogeneously distributed in space and time. Many plant species are able to respond to this heterogeneity by modifying their root system development, such that they colonize the most resource-rich patches of soil. The complexities of these responses, and their dependence on the implied ability to perceive and integrate multiple external signals, would seem to amply justify the term 'behaviour'. This review will consider the types of behaviour that are elicited in roots of Arabidopsis thaliana by exposure to variations in the external concentrations and distribution of two different N compounds, nitrate and glutamate. Molecular genetic studies have revealed an intricate N regulatory network at the root tip that is responsible for orchestrating changes in root growth rate and root architecture to accommodate variations in the extrinsic and intrinsic supply of N. The review will discuss what is known of the genetic basis for these responses and speculate on their physiological and ecological significance.