This study investigated the extent to which vocal production in wild Japanese macaques, Macaca fuscata yakui, is flexible. Japanese macaques frequently exchange coo calls with other group members to maintain auditory contact. When a coo call is emitted but no other members respond within a short interval, the same monkey often emits another call repeatedly. This study focused on these two successive coo call sequences. First, the sequences of eleven females in a free-ranging group were recorded and analyzed. Comparisons of the acoustic properties between the initial and repeated coo calls revealed that the repeated call was higher in fundamental frequency and longer in duration than the initial call. Next, playback experiments were conducted to examine the efficiency of the exaggerated acoustic features of these repeated coo calls. Eight pairs of initial and repeated coo calls were prepared from eight female monkeys as the playback stimuli, and played back to the eight subject females. Comparisons of subject responses to the initial and repeated coo call stimuli revealed that the repeated call stimuli elicited more frequent vocal responses. These field observation and playback results suggest that monkeys might be able to modify the acoustic properties of vocalizations according to the preceding context so as to elicit a more effective vocal response from other group members. The pattern of vocal interactions among wild Japanese macaques is therefore likely to be flexible and to some extent context-sensitive.