Inside the organism, serotonin is obtained through tryptophan hydroxylation and decarboxylation, and direct tryptophan decarboxylation leads to tryptamine (Figure 2) (De Research ArticleAbstract | The current study aimed to determine the serum serotonin in 10 dogs, separated in two different emotional states (calm, aggressive). According to the specialty literature, at the aggressiveness moment, the serotonin value drops. The aggressiveness manifestation in dogs is induced by several factors, among which feed occupies a special role, hypo-hyper-protean ratios influencing the behaviour. The results indicate that individuals fed with hyper-protean ratios presented lower serotonin values as compared to dogs receiving hypo-protean ratios.