The mechanism behind masseter muscle pain, a major symptom of temporomandibular disorder (TMD), has remained poorly understood. Previous report indicates that adenosine triphosphate (ATP) is involved in the masseter muscle pain development, but the role of its hydrolysis product, adenosine diphosphate (ADP), remains uncertain. Consequently, this study aimed to elucidate the ADP role derived from the sustained masseter muscle contraction in the masseter muscle pain development. The right masseter muscle was electrically stimulated daily by placing electrodes on the muscle fascia, inducing strong contraction and mechanical allodynia. This led to an increment of the ATP release from the masseter muscle and a consequent increase in ADP produced by the hydrolysis of ATP. The mechanical allodynia was suppressed by intramuscular P2Y