Using a lead precipitation technique, we investigated the cytochemical localization of adenosine triphosphatase (ATPase) and acid phosphatase (APase) in the minor veins of young leaves and in the sink and source regions of leaves undergoing sink-source transition in Vicia faba. The leaves undergoing sink-source transition were determined by the 6(5) carboxyfluorescein tracing. The plasmalemma and cell wall of transfer cells and the differentiating xylem elements in the young leaf showed intensive reaction products of ATPase and APase. The reaction products of ATPase and APase were weak in the plasmalemma of both transfer cells and sieve elements of the minor veins in the sink region. We observed a large number of ATPase and APase reaction products in the plasmalemma of both sieve elements and transfer cells. Thus, dynamic changes in the distribution of ATPase and APase occur at different developmental stages of minor veins. The role of ATPase and APase in apoplastic loading and differentiation of sieve and xylem elements is discussed.