Despite the increasing availability of more-sophisticated methods for simulating rainfall–runoff events, the 'rational method' continues to be used as a design tool in many municipal engineering offices. This paper examines the basic assumptions of the method and shows how hydrographs from impervious areas can be accurately simulated by a simple convolution process using the rectangular response function implied in the rational method. The use of the dynamically varying response function appears to give improved results. Moreover, in the example illustrated, routing of the runoff through a hypothetical reservoir appears to be unnecessary.For pervious areas, a method is suggested whereby the runoff coefficient is varied as a function of the time-dependent storage potential in the soil. For events with modest rainfall abstractions, the method appears to give good agreement with observed runoff hydrographs, but inclusion of a routing process through a cascade of reservoirs seems to be necessary in this case. Key words: computer, design, hydrology, rainfall, rational, runoff.