This paper represents Part I of a two-part study examining preservice teachers' development of a personalized, research-based Science Teaching Rationale (STR). Researchers have historically documented the application of the "rationale paper" (Clough, 1992; Veronesi, 1998) using qualitative methodologies. Since the rationale paper continues to receive attention at conferences (e.g., at the annual conference of the Association for the Education of Teachers in Science) and grow in popularity as a term assignment among science educators, further study was warranted. This quantitatively dominant study, using the "Self-Report Instrument for the STR," is the first work of its kind on the STR. It adds to the body of understanding regarding its impact on the thinking and dispositions toward science teaching of preservice elementary teachers. Discussion on development, implications, and elements of instrument reliability and validity are included. (Contains 3 tables.)