In this study, ultrastructural changes on oviducts of guinea pigs during postnatal development were investigated. The study population was consist of 56 female guinea pigs which were divided into seven groups according to age; 1 days, 2, 4, 6, 12, 18 week and older than 18 weeks. Each group contained 8 guinea pigs.

The oviduct was lined with uniform layer of simple columnar epithelium. In the oviductal epithelium, the cytoplasm of non-ciliated cells had stained more densely with toluidine blue than that of the ciliated cells. The myosalpinx has been consisted of an inner longitudinal layer and an outer circular layer.

At 4^th and 6^thweeks, it was established increase of height mucosal folds in the tunica mucosa and in the diameter of the oviductal lumen.

Most of the epithelial cells of oviduct were either ciliated cells or secretory cells. The most conspicious characteristic of the secretory cells, based on their staining property, was the presence of two types of secretory granules: those with a homogenous electron-dense matrix and those with an electron-lucent matrix.

Cilia and microvilli have been extended from the surface of the cell into the lumen. The mitochondria of the ciliated cells are numerous, located mostly in the supranuclear region.