The measurement of an elementary mental function of a person can be obtained in many conditions. Several procedures are available, e.g. tests, reaction time measurement, psychophysical methods, etc. However, in some cases none of them seems appropriate. For instance, it is hard to find a suitable method to confirm the deficit of the so-called mental number line in developmental dyscalculia (developmental problem of numerical abilities). The selective load method introduced in this paper is a reaction time measurement method based on a special design. The validity of the method is verified on the example of numerical tasks. Solving numerical problems requires several mental functions, according to the tasks: e.g. semantic representation of quantities, verbal retrieval, executing operations. According to the validation the selective load method is able to isolate elementary functions involved in the solution of the task. Compared to previous procedures, the selective load method provides a more valid method for measuring the selected functions, offering new possibilities in basic research, and in diagnosis of developmental and acquired deficits.