Résumé : Buyer–supplier (B–S) relationships are dynamic. Nevertheless, research on external supply chain integration (SCI) has largely adopted a static-snapshot perspective and has failed to grasp SCI's dynamic reality. The scant research devoted to SCI's dynamic perspectives suggests that the strength and scope of ties between buyers and suppliers differ based on integration levels. However, ambiguity persists regarding the relational behaviors exhibited across integration levels, as well as the stimulants that motivate firms to evolve their relationships. With buyers increasingly relying upon suppliers (and vice-versa) and compounded by the knowledge that nearly half of SCI initiatives end up as failures, the need for a holistic framework that takes into account the underlying dynamics of relational behaviors and stimulants cannot be overstated. Using a mixed-methods approach, six relational behaviors that differ in their saliency across SCI levels are identified, and eight stimulants that motivate B–S relationships to evolve are extracted. Specifically, we note that once a stimulant is activated, it remains influential across SCI levels, though its potency may vary. Our research significantly extends the contemporary understanding of SCI by (i) providing a holistic framework to comprehend SCI from a behavioral perspective, and (ii) noting that the highest integration levels do not necessarily typify successful B–S relationships. In doing so, we combine an evolutionary perspective with a relationship portfolio perspective for examining B–S relationship dynamics, thereby offering a comprehensive view to yield finer-grained insights into B–S relationship dynamics.