Mast cells (MCs) are important object in experimental acupuncture due to their putative involvement in local reactions to needling. In the rat, they are shown to contain in their granules, among other tissue mediators, serotonin, also called 5-hydroxytryptamine (5-HT). The aim of this study is to examine the normal distribution of 5-HT-containing MCs in soft tissues of Zusanli (ST36) acupuncture point (acupoint) and their morphological changes caused by experimental acupuncture. We observed 5-HT-immunopositive MCs in the tissues and in the vicinity of the needle tract formed after acupuncture. As a result of acupuncture needling, the tissue integrity is disrupted and certain folds are formed in the direction of the needle tract. Connective tissue in the vicinity of the needle tract gets compressed and displaced, together with the 5-HT-immunoreactive MCs seen there. Some of those 5-HT-immunopositive MCs showed signs of degranulation with numerous discharged granules, some of them found at a considerable distance form the cell. Furthermore, 5-HT-immunopositive MCs are unevenly distributed in soft tissues of ST36 acupoint. Larger numbers of 5-HT-containing MCs were visualized in subcutis and dermis, compared to the observed in striated muscles. Placing the acupuncture needle into the rat skin caused a formation of an apparent needle tract, tissue displacement and degranulation of 5-HT-immunopositive MCs. The demonstrated serotonin release by means of MC degranulation might be involved in the local tissue response to acupuncture.