The paper contains data on the level of play and its effect on arbitrary behaviour in modern preschool children and preschoolers of the forties and fifties of the last century. Comparison reveals considerable deterioration of role play in modern children. The study reproduced the widely known experiment first conducted by Z.V. Manuilenko in 1948. Comparison of data on preschoolers of the middle of the 20th and the beginning of the 21st centuries also reveals considerable deterioration of arbitrary behavior both in play and non-play situations. The data obtained suggest that primitive underdeveloped play cannot set the zone of proximal development and conform to the status of the leading activity.