By means of elements of the structure of activity, i. e. individual performances and operations, the complexity of teaching assignments can be determined. Individual performances and operations must then be classified as determining and executing performances and operations. The executing performances and operations must be considered primarily in tasks of the assignment character (being worked out by the pupils by acquired algorhythm), the determining operations and performances must be considered, next to the executing operations and performances, in the working out of teaching assignments of a problem character. The complexity of the teaching assignments can be influenced, i. e. by reducing or by increasing auxiliary or overflowing (erroneous) information in the assignment. The given information is further classified into algorhythmic, having a dead-line and factual (the finished result) and is related to general activity, performance or operation and, as shown by the outcome of research, influences in various ways the complexity of a definite teaching assignment. Mastering the information on the determination and influencing the complexity of the given assignment by the chemistry teachers ensured, that active teaching methods started to be used to a greater measure in teaching practice, because the given teaching assignments corresponded to the knowledge, skills and the abilities of the pupils.