Functional similarity of catchments implies that we are able to identify the combination of processes that creates a similar response of a specific characteristic of a catchment. We applied the concept of functional similarity to the export of NO3--N from catchments situated within the Turkey Lakes Watershed, a temperate forest in central Ontario, Canada. Despite the homogeneous nature of the forest, these catchments exhibit substantial variability in the concentrations of NO3--N in discharge waters, over both time and space. We hypothesized that functional similarity in the export of NO3--N can be expressed as a function of topographic complexity as topography regulates both the formation and flushing of NO3--N within the catchment. We tested this hypothesis by exploring whether topographically based similarity indices of the formation and flushing of NO3--N capture the observed export of NO3--N over a set of topographically diverse catchments. For catchments with no elevated base concentrations of NO3--N the similarity indices explained up to 58% of the variance in the export of NO3--N. For catchments with elevated base concentrations of NO3--N, prediction of the export of NO3--N may have been complicated by the fact that hydrology was governed by a two-component till, with an ablation till overlying a basal till. While the similarity indices captured peak NO3--N concentrations exported from shallow flow paths emanating from the ablation till, they did not capture base NO3--N concentrations exported from deep flow paths emanating from the basal till, emphasizing the importance of including shallow and deep flow paths in future similarity indices. The strength of the similarity indices is their potential ability to enable us to discriminate catchments that have visually similar surface characteristics but show distinct NO3--N export responses and, conversely, to group catchments that have visually dissimilar surface characteristics but are functionally similar. Furthermore, the similarity indices provide a potentially powerful method to scale and generalize NO3--N export responses to other regions. ¿ 1998 American Geophysical Union