Although employees' willingness to cooperate is acknowledged as a critical success factor for post-M&A (merger-and-acquisition) integration, we still know little about the psychological mechanisms that lie beneath employees' cooperative attitudes and behaviors in this context. Building on the premises of fairness heuristic theory, this longitudinal study explores how the relative importance of distributive and procedural justice judgments for employees' willingness to cooperate shifts over time. We suggest that when employees lack justice-relevant information on both distributive and procedural aspects of decisions, they will use another temporary heuristic to reduce uncertainty by scrutinizing the M&A-related cooperative behaviors of authority figures. We test our hypotheses on data from a four-time repeated cross-sectional survey of employee responses in a post-M&A integration process. The findings provide important insights into how merging firms can enhance employees' willingness to cooperate through the subtle exercise of justice and exemplarity.