Background: Stress has been known as one of the various causes of hair loss. However, the specific mechanism of acute stress-induced hair loss remains unknown. The previous studies revealed that mast cell is involved in neurogenic skin disorders. Therefore mast cell is suspected as a link between stress and alopecia. Objectives: This study investigated the effect of acute stress on the hair growth, and its mechanism. Methods: Rats in the stress group were immobilized using rodent restrainer devices and were deprived of all food and water for 24 hours. The rats in the control group were separated into each individual cage, but not immobilized. After 24 hours, cortisol level and the various cytokines in serum were measured and back skin biopsy with alcian blue staining, immunofluorescence staining and western blot of mast cell tryptase were conducted. In addition, we measured the effect of rat stress-serum on human dermal papilla cells (hDPCs) proliferation in-vitro by MTT assay. Results: The restraint for 24 hours significantly increased cortisol level. In addition, IL-6 and IL-1ß were up-regulated after the restraint. Acute stress increased the number of mast cells and the expression of mast cell tryptase. The stress-serum treatment reduced significantly the number of viable hDPCs. Furthermore, cell cycles of treated hDPCs were in G1 phase in more portion than of the control cells. Conclusion: These results suggest that acute stress can inhibit hair growth by mast cell activation.