The paper is an attempt to determine mutual relations between the length and type of activity related to intellectual work and the degree of mental fatigue and the heart rate reserve. The tests were carried out on the group of 25 persons, who through realization of particular training stages concerning operation of modern farm tractors through special logical tests and measurement of the heart rate generated information on the mental fatigue and stress during the experiment. It was found out that along with the increase of the duration of intellectual work, the time necessary for accomplishing a logical task and number of mistakes increases. It was also reported that with each test of a similar logical structure, the stress factor decreased and the process of learning took place in case of learning persons. Thus, during the last measurement, the investigated group obtained relatively better results than in the previous test.