The enzyme, carbonic anhydrase (CA), has been studied by an histochemical procedure during the postnatal development of the rat epididymis. Its activity first appeared at Day 14 in the early differentiating cells, i.e. the narrow cells, and only in them. These cells, found in the initial segment and the head of the adult epididymis, showed considerable activity. At Day 35 they disappeared in the other segments (corpus and tail), while the light cells which became apparent immediately showed more discrete CA activity. No carbonic anhydrase activity was found in the seminiferous epithelium, the testicular interstitial tissue or the rete testis.