Synchronization of behavior between individuals has been found to result in a variety of prosocial outcomes. The role of endorphins in vigorous synchronous activities (Cohen, Ejsmond-Frey, Knight, & Dunbar, 2010) may underlie these effects as endorphins have been implicated in social bonding (Dunbar & Shultz, 2010). Although research on synchronous behavior has noted that there are 2 dominant phases of synchrony—in-phase and antiphase synchrony (Marsh, Richardson, Baron, & Schmidt, 2006), research on the effect of synchrony on endorphins has only incorporated in-phase synchrony. The current study examined whether both phases of synchrony would generate the synchrony effect. Twenty-two participants rowed under 3 counterbalanced conditions—alone, in-phase synchrony and antiphase synchrony. Endorphin release, as measured by pain threshold, was assessed before and after each session. Change in pain threshold during the in-phase synchrony session was significantly higher than either of the other 2 conditions. These results suggest that the synchrony effect may be specific to just in-phase synchrony, and that social presence is not a viable explanation for the effect of synchrony on pain threshold.