This longitudinal study of changes in play behavior in preschool children reports decreases in less mature forms of play and increases in more mature forms of play over time. The study was designed to test the assumption that the play of young children develops in a progressive manner. Ten 3-year-olds and ten 4-year-olds were observed during free play each day for four 3-week periods over 3 months. The coding of behavior was based on Parten's categories of social play (unoccupied, solitary, onlooker, parallel, group) and Smilanksy's categories of cognitive play (functional, constructive, dramatic, games with rules). Two additional categories included reading (or being read to) and active conversation. Children showed a decrease in the amount of time spent in unoccupied, onlooker, solitary-functional, and all functional play, and an increase in the amounts of time spent in reading, games, parallel-dramatic, all group, and all dramatic play. Only the games with rules category showed a differential age-by-time interaction. Alternative explanations of the data are discussed. Data on age and sex differences are also reported. Two tables summarizing results are included. (BH)