The norm of reciprocity shapes social interactions over time and should therefore be analyzed using longitudinal designs. This paper examines the temporal dynamics of reciprocity in the setting of legislative cosponsorship in the 113th U.S. Congress (2013–2015). Rather than aggregating cosponsoring events into network snapshots, cosponsoring events are ordered in time and reflect an event sequence of members supporting bills at distinct points in time. The norm of reciprocity is tested to see if it affects (a) new collaboration clusters emerging over time or (b) the time-to-cosponsoring. Findings suggest that receiving cosponsoring support recently, leads to future collaborations. However, this finding holds only for Republican members of Congress. Furthermore, reciprocity demands do not expedite cosponsorship support and members do not react faster to favors they received, calling mitigating effects of reciprocity on collective action into question.