The distribution of substance P immunoreactive(SP-IR) nerve endings in the nasal gland of the guinea pig was studies by immunohistochemical elecron microscopy using avidin-biotin-peroxidase complex(ABC) method and pre-embedding technique. The SP-IR nerve endings were located in the region between the interdigitated cytoplasmic folds of the acinar cells and along the basal surface of the acini, as well as the intercellular space of proximal ducts. The acini which closely contacted with SP-IR nerve endings were serous in type. These findings suggest that substance P may contribute to the regulation of serous glandular secretion in the guinea pig nassal mucosa.