Mink, as carnivorous fur animals, are especially exposed to stress factors during mating, gestation, parturition, and rearing of the young that manifests as aggressive behaviour. So far, preparations applied on mink farms to relieve stress have shown low efficacy. Therefore, we aimed at determining whether the Relaxer feed supplement, prepared from valerian (Valeriana officinalis L.) and passionflower (Passiflora incarnata) extracts, can be applied to reduce the negative effects of stressors. The study was conducted on mink (Neovison vison) of the brown variety on a fur farm (Poland). The study was divided into two stages investigating the effect of the Relaxer additive on adult mink during the mating season (1%, 1.5%, 2% additive) and on young mink weaned from their mothers (1.5%). For all mink groups basic haematological and morphological blood parameters were determined. The animals’ condition and health were observed, and behavioural observations were made using a temperament test. The results of blood parameters of mink showed no negative impact of the additive on the animals’ health. Statistical differences were observed in red blood cell counts. Analysing the white blood cell count parameters from the first stage of the study, a decrease in white blood cell (WBC) count was observed in the blood of animals of groups K and D1 in successive intakes. In this stage, the lowest cortisol levels were obtained in groups D1 and D2. Analysing the red blood cell parameters of animals at stage II, statistically significant differences were observed. Smaller differences were observed for the studied parameters of the protein-cell image of mink blood. A lower percentage of animals with extreme behavioural types (aggressive and fearful) was found in the experimental group. The feed addition of a mixture of herbs significantly influenced the stress behaviour of mink. Improved behaviour was observed which resulted in improved animal welfare and optimised production parameters. The optimal proportion of the additive in the feed was set at 1.5%.