Almost 300 years old pedagogical work of J. A. Comenius, teacher of peoples and humanist, remains a rich source of the current pedagogical problems and pedagogical knowledge even in the present-day world of science, technology and great social struggles. In our study, which by no means claims to be complete, we are at least trying to show in an outline why and in what the great work of J. A. Comenius is permeated by an expressive humanizing factor: mental hygiene, in his terminology care for the spirit or the mind. We hold the view that the vitality and greatness of his work is fostered by the very fact that all of it is to an unusual extent pervaded by the care for the physical health and especially for the mental health of the youth being brought up. Both kinds of this expressive care are understood by Comenius as forming a unity, which is quite the same as the modern present-day conception. If Comenius looks upon education as an effective humanizing factor, one cannot help seeing that it is just this all-round care for health that is the factor helping the effectiveness of education. It is just this humanistic approach to the child that, makes Comenius’ school and his whole work a school of humanity, a joyful school of play and serious work, a school of knowledge, training feeling and character. At the same time it is a pleasant school, in which the pedagogical process follows and develops the needs of human nature and becomes, according to Comenius, spontaneous and even playful. And if the pedagogical process is going on like this, it simultaneously becomes a process of reinforcing and developing mental health. By pointing out some characteristic features of Comenius’ conception of the pedagogical process, we show their overt or more or .less concealed mentally hygienic concomitant. It is most interesting that many psycho-hygienic principles that can be pointed out by analysing Comenius’ pedagogical work are still recognized today, even though modified and made more precise by the subsequent development of sciences and the needs of modern times. Comenius shows how school, by increasing its care for physical and mental health, becomes an attractive and many a time even entertaining workshop of humanity; he shows how a healthy mind, which is reinforced by such a school, equips pupils safely for life. These mentally hygienic undertones of the pedagogical work of Comenius concern above all the prevention and the “cure” of the defects of character and move rather in the field of moral defects, where ' remedial treatment even today is most likely to be represented by specially adapted curative educational influences. We (have tried to assess and interpret here only some of the intentions of Comenius’ pedagogical work from the point of view of mental hygiene. His great work conceals, of course, a great wealth of problems involving the hygiene of the pedagogical process, and would deserve a profound, systematic study as well as confrontation with present-day theories of the conception of mental (hygiene, with the methods of the prevention of mental diseases as well as with the modern treatment of these diseases. On the occasion of the tercentenary of the death of the great 'humanist teacher of peoples we have tried to repay at least partly this debt to his ingenious work but also to our socialist school. For it is joist now, in the conditions of general haste of contemporary life, that it is more than ever necessary to pay increased attention to the development of mental health, when the care for physical health seems to be on the whole satisfactory. The work of Comenius, as we have tried to show, has remained modern and is still living just in its emphasis on the care for mental health; in this respect both the theory and the practice of the current modernization of our socialist school may find a lot of useful information in it.