The objectives of this study were (1) to observe and describe the social quality and the cognitive type of spontaneous play exhibited in a group of 20 children in a child care setting during two age periods, toddler (14-30 months) and preschool (30-48 months), and (2) to chart the developmental trends using a model of play complexity derived from the theoretical constructs of Piaget and Parten. Comparisons of sex differences also were made. Observational data were collected on videotape and coded according to sequenced categories of social quality (Onlooking-Solitary, Parallel, Associative, and Cooperative Play) and cognitive type (Practice, Symbolic, and Games). Results supported the sequences outlined by Piaget and Parten, although the amount of associative and cooperative play at older ages was not as great as Parten reported. Play complexity increased as age increased. Social and cognitive complexity both were predicted by chronological age and sex, as was overall complexity. Sex differences were evident for cognitive complexity but not for social complexity. The social quality measure was a more useful predictor of play complexity than the cognitive type measure. (Author/SS)