In the study of ability to plan actions took part 97 healthy mono- and dizygotic twins 5-6 years of age, who performed "The tower of London" task, which included 16 trials with different number of moves. For 67 participants retest validity was evaluated for the interval of 3,5 weeks between sessions. There were no age-specific differences in the number of successful trials but the mean speed of correct trials increased with age. The authors argue that planning efficacy in preschoolers increases due to the development of inhibition processes and operative memory. There was low correlation between test and retest results, may be due to the differences not only in abilities to plan but also in learning abilities.