Castration of dogs is often conducted as a preventive measure against diseases and undesirable behaviour. While female dogs are preferably neutered for medical reasons, owners of male dogs hope for an improvement in behavior. Although there is a lack of scientific knowledge on this subject, neutering is often conducted to get a more trainable and less aggressive dog. The aim of this study is to examine castration from an ethological perspective and to reveal possible behavioural changes after castration related to breed. An online study was conducted consisting of two different questionnaires about the dog’s personality. A total of 242 dog owners participated in the study concerning specific behaviour problems (n = 130 intact and 112 neutered males). Another 211 owners of males (n = 115 intact, 96 neutered) completed the questionnaire about personality traits (dog sociability, trainability, emotional stability and extraversion) based on Turcsán et al. 2011. Four breed categories were formed following Parker et al. (2017): shepherds, retrievers, terriers, and hunting Dogs. Our study reveals that intact males are bolder than neutered males. Intact males have lower aggression scores than neutered ones. Castrated males show significantly more panic behavior (multinomial logistic Regression, p = 0.04). Intact shepherds are bolder than neutered ones (Mann-Whiney-U-Test: p = 0.03) and intact terriers are bolder than neutered terriers (Mann- Whitney-U = Test: p = 0.04). Intact terriers scored lower for aggression than neutered ones. With these results and the data of previous studies, we question castration to modify behavior, as a measure for reproductive control and as a preventive measure against diseases.