Although previous research has explored the effects of various environmental influences on young children's play, the influence of toys has rarely been examined. This paucity of toy studies is due to a lack of a scientifically constructed observation system to evaluate the impact of play materials across developmental domains. The purpose of this study was to develop and test the reliability and validity of such an instrument. An eight-item, five-point rating system was constructed, based on previous toy research. To establish inter-rater reliability, two researchers independently used the instrument to rate the effects of 23 toys on the spontaneous play of 16 preschool children. There was a high level of agreement between observers (r = .81-.88, across items). A factor analysis was conducted to identify clusters of items that measure like dimensions of play effects; three distinct factors were identified: thinking/learning, creativity/ imagination and social interaction. To establish validity, the instrument was used to rate five common toys that could be predicted, based on earlier work, to have distinct influences on play. The instrument was found to discriminate among these toys in ways that are consistent with previous research and that make theoretical sense. The uses of this instrument in both future research and classroom practice are discussed.