ABSTRACT. This study was carried out to investigate the effect of ovariohysterectomy on vocalization during territorial aggression of German Shepherd dogs. Sixteen clinically healthy dogs of 5 to 10 months old were assigned randomly to one of two groups: ovariohysterectomy or control. Their behaviors and vocalizations induced by the approach of a stranger with a strange dog were recorded using digital camcorder at four and five months after surgery and were analysed. When territorial aggression was induced, dogs in ovariohysterectomized group showed more offensive territorial aggression. The bark was the most frequent vocalization. In this study, the average number of barks was 45 times in ovariohysterectomized group, and 26 times in the control group. The pitch of vocalization was significantly lower in ovariohysterectomized group than control group. First formant, second formant, third formant, and fourth formant frequency in ovariohysterectomized group, which represent the degree of sound energy in specific frequency, were lower than those of control group. Ovariohysterectomy of bitches may influence the frequency of aggressive vocalization and affect the acoustic feature of dogs' vocalization. Analysis of vocalization could be a useful method of evaluating the dogs' intention.