The application of existing desktop geographical information systems (GIS) to the assessment of accessibility by public transport has been investigated. Two approaches to the measurement of accessibility aggregate accessibility measures and the space-time geography framework are described. It is suggested that isochrones (lines of equal travel time) are a natural way to combine these approaches in a GIS setting. A desktop GIS application has been developed which can automatically generate isochrones for travel by public transport. The application successfully copes with the complexity of the public transport of a large city (Glasgow), comprising bus, rail, and underground services. Journeys may include an initial walk to a public transport stop, several interchanges between services of the same or different modes, and a final walk. Options provide for plotting isochrones for journeys by bus only or train only; and for exact or approximate isochrones, the latter generated more quickly. The application is described in some detail, and assessed. Sample outputs are presented, including an example involving further processing of isochrones to produce a constrained accessibility map, which demonstrates the generality of this approach.