The Model-based Analysis and Reasoning in Science (MARS) project is involved in developing model-centered curriculum modules for middle-school science. MARS instruction focuses on a variety of physical, pictorial, and symbolic representations of theoretical entities, providing tangible objects that students can use to think and talk about abstract concepts and links between concepts. This article examines the impact of the mass unit, the third unit of a sixth-grade curriculum module, implemented in a public school during the 1993 -1994 school year. Following an overview of the unit itself, three sections describe its impact as revealed by three aspects of evaluation: (1) classroom observations of opportunities to learn afforded students through instructional activities and interactions; (2) examination of students' written work, including worksheets and free-response test performance; and (3) analysis of protocol data from individual interviews conducted at the end of each of the three main sections of the curriculum. The article concludes with a discussion of specific implications for the MARS curriculum and general implications for science instruction.