The present study explored the effects of three different activity conditions on three attentional functions: alerting, orienting, and executive control. A group of highly experienced cyclists performed the Attention Network Test-Interactions (Callejas, Lupiáñez, & Tudela, 2004) at rest, during moderate aerobic exercise, and during intense aerobic exercise. Results indicated that aerobic exercise accelerated reaction time and reduced the alerting effect compared with the rest condition. However, aerobic exercise did not modulate the functioning of either the orienting or the executive control attentional networks. No differences in reaction time or attentional functioning were observed between the two aerobic exercise workloads. The present results suggest that moderate aerobic exercise modulates the functioning of phasic alertness by increasing the general state of tonic vigilance.