The histochemical and biochemical distributions of gamma-glutamyl transpeptidase (gamma-GT) were investigated in the epididymis of rats during fetal and postnatal development. In the epididymal homogenates, gamma-GT activity was detected on the fifth day after birth. A sharp increase was observed after 30 days of life in the caput homogenates. Moderate levels of the enzyme were found in the cauda epididymis. Gamma-GT is histochemically detected from the 15th day of gestation in Wolffian ducts and in 17- to 18-day-old fetuses in newly differentiated epididymal tubules. Enzyme activity, was associated with the plasma membranes (apical, lateral, and basal), was preponderant on the apical part of the epithelial cells. During the first 15 days of the postnatal life, the histochemical reaction intensities were identical from the caput to the cauda epididymidis. From the 18th day onwards, enzyme activity decreased in the corpus and in the cauda, while gamma-GT increased in the caput epididymidis, and a strong activity was found on the apical surface of epithelial cells. Weak or moderate gamma-GT activity of spermatozoa in the caput tubules, increasing steadily from caput to cauda epididymidis, suggests that gamma-GT may be related to the functional maturation of spermatozoa.