Unlike the existing difficulties in determining typological peculiarities of the higher nervous activity on the basis of the strength, equilibrium and mobility of the processes of excitation and inhibition (or signalling systems characterizing the higher nervous activity of man), it is possible to show, by an analysis of the data gathered in this paper, that the same goal can be reached relatively easily by means of standardized differentiation tests. This is evident from the results of experiments carried out with 200 Nursery and Basic School pupils and 24 Special School pupils; the pupils were to differentiate 15 positive and 15 negative stimuli (pairs of letters of the alphabet or simple geometrical drawings). The stimuli used were the same for all the pupils of the age-groups that were compared. The data obtained show that the Basic School pupils differentiated better on an average than the Special School pupils of the same age and also the performance of the older pupils of the Basic School was better than that of the younger ones. The older pupils differ from the younger ones by stronger positive and negative reflexes, as a result of which they have a greater ability of generalization and abstraction. The indices of differentiation used for a numerical expression of the pupils’ performance and their division into three types (whose theoretical drawbacks as well as practical advantages we are fully aware of) show that the performance of the pupils in the extreme cases of the strong and the type (i. e. pupils with indices of differentiation higher than 0.75 or 0.25 and lower) maintain their significance in all age-groups. Also he indices of the middle types show change as the pupils grow older and more experienced. To what extent these changes do occur will be shown by longitudinal research The stage development of the ability of differentiation, which is also confirmed by similar findings of other authors, is of a collective nature, fully explicable by the organization of the school without prejudice to the smooth mental development of individual pupils. The diagnostic significance of the differentiation tests is obvious, especially if all the necessary measures (following the example of the tests) are taken to further increase their reliability.