The purpose of this study is to examine what factors must be satisfied to maintain a continuous and stable contractual relationship—that is, a partnership—between manufacturers and parts suppliers, which is emphasized in the adoption of the JUST-IN-TIME (JIT) system. A synthesis of prior research indicates that structural, behavioral, and information-exchange variables create a favorable environment for the successful adoption of JIT, and that cooperation between manufacturers and suppliers is strengthened under the influence of these variables. The two central research objectives of this study were: first, to analyze the effects of cooperative partnerships in JIT purchasing on the achievement of JIT objectives; and second, to investigate how structural, behavioral, and communication factors influence the formation of cooperative relationships between manufacturers and suppliers. The findings suggest that factors such as cooperation, goal congruence, and equity between buyers and suppliers serve as key success factors for achieving positive outcomes in JIT purchasing relationships.