Vygotskian theory and empirical evidence suggest that children's private speech and pretend play contribute to their development of motivational processes. Given current U.S. preschool expansion, and resurgent debates over the merits of play-based vs. non-play-based approaches to early childhood education, this study conducted an experimental investigation of the relative impact of these contexts on preschoolers' private speech and mastery motivation (performance and persistence). 38 preschool children engaged in a challenging fishing activity in two experimental conditions (playful and non-playful) simulating pedagogical and motivational (intrinsic vs. extrinsic) characteristics of common preschool settings. Private speech was categorized as cognitive, motivational, metacognitive, playful or partially internalized, and the emotional valence of private speech was marked as positive or negative. Results indicated that preschoolers in the playful condition displayed higher mastery motivation than preschoolers in the non-playful condition. Children in the playful condition used more frequent private speech, including more frequent cognitive, playful, and positively valenced private speech. Mastery motivation was positively correlated with playful, partially internalized, and positively valenced private speech, but negatively related to motivational private speech. Mastery motivation components (performance and persistence) related to different types of private speech. Performance related positively to metacognitive private speech and negatively to motivational private speech. Persistence related positively to playful private speech. The playful condition elicited private speech categories that were associated with higher motivation levels. Findings support the use of playful and play-based pedagogy in early childhood education, and teacher modeling of motivationally beneficial forms of private speech. (author abstract)