Spelling mistakes of the pupils of the 3-d - 10-th forms are unevenly distributed along the length of a predicative unit (an elementary sentence). In short predicative units consisting of 2 or 3 words each subsequent graphic word carries more mistakes than each preceding one. In longer elementary sentences besides the peak of mistakes in the last word there can be observed spelling mistake peaks in the middle. The number of the peaks in the middle of a predicative unit consisteng of 4-14 words depends on its length and the kind of written speech activity (composition, reproduction, dictation). Spelling mistakes get accumulated in such parts of elementary sentence where operative memory, responsible for its composition, experiences the greatest strain. As a result "breakdowns" take place in its activity finding their expression in the peaks of mistakes.