BACKGROUND Glutathione S-transferases (GSTs) are a family of isozymes that catalyze the conjugation of the tripeptide, glutathione, to various electrophilic compounds. The major GST in the pi class is GST-P, a homodimer of the Yf subunit, also known as Yp or rat subunit 7. This subunit is found in high concentrations in the epididymis and has recently been immunolocalized within epithelial principal and basal cells of the epididymis. METHODS In the present study we examine in groups of animals fixed in Bouin's fixative for light microscopy and in 4% paraformaldehyde and 0.5% glutaraldehyde in phosphate buffer for electron microscopy, the pattern of immunostaining for the Yf subunit of GST-P in the testis, efferent ducts and epididymis at various ages after birth. RESULTS In the epididymis, on postnatal days 7 and 15, an immunoperoxidase reaction was localized exclusively to the apical and supranuclear regions of the undifferentiated columnar epithelial cells of the entire epididymis. By day 21, a dramatic change had taken place. In the initial segment, intermediate zone and proximal caput epididymidis, the columnar cells showed a distinct checkerboard-like staining pattern with cells ranging from being intensely reactive to unreactive. In contrast, principal cells of the distal caput, corpus, and proximal cauda epididymidis were weakly reactive. By day 28 the ratio of reactive to unreactive cells in the initial segment, intermediate zone, and proximal caput epididymidis was higher. By day 39, the differentiated columnar epithelial cells, referred to as principal cells, took on their adult staining pattern in the proximal and middle areas of the initial segment as well as the corpus and proximal cauda epididymidis where they were slightly reactive; in the distal initial segment they were strongly reactive. At day 49, principal cells in the intermediate zone and proximal caput became intensely reactive, while showing a distinct checkerboard-like staining pattern in the distal caput; similar observations were made for tissues taken from 56 and 90-day-old animals. Basal cells also showed a variable staining pattern in the different epididymal regions as a function of age. At day 21, when they first appeared, they were unreactive except for an occasional reactive cell in the corpus region. At day 28, only in the corpus epididymidis were many basal cells seen to be reactive. By day 39 the more numerous basal cells of the corpus and proximal cauda epididymidis were intensely reactive and remained so into adulthood. In these regions, basal cells appeared as dome-shaped cells (days 21, 28, 39), but then gradually flattened out and exhibited processes (days, 49, 56, adults) which collectively appeared to envelop the base of each tubule in a mesh-like network. The change in basal cell shape in each region coincided with the arrival of fluid and spermatozoa into the lumen (corpus day 49, proximal cauda day 56). In other epididymal regions, basal cells at day 28 were mostly unreactive. However, there was a gradual increase in the number of reactive basal cells of these regions between day 39 and 56. CONCLUSIONS The present results thus demonstrate a dramatic change in the immunostaining pattern for the Yf subunit of GST-P during postnatal development for both principal and basal cells along the epididymis. Such results suggest that different factors play a role in the regulation of the expression of the Yf protein, not only in different epididymal regions, but also in different cell types during postnatal development.