Pant hoots, a type of long-distance calls of chimpanzees (Pan troglodytes), were played back to two male chimpanzees in a group of seven captive individuals to determine if chimpanzees would modify those vocalizations in response to strange males. Subjects emitted pant hoots with higher rates of delivery, shorter duration of buildup, and lower minimum fundamental frequency of climax when they were presented with pant hoots of strangers than when they produced the calls spontaneously. Considering the direction of acoustical change, we concluded that the rate of delivery, duration of buildup, and minimum frequency of climax might be associated with the underlying emotional states of the callers rather than call matching. Individual difference between two subject males was significant in minimum frequency and duration of climax and in average frequency of call, which appears to reflect differences of the caller's age and social status. These results suggest that different acoustic variables relate to within- and interindividual differences of these vocalizations.