Summary Measurement of electrical resistance across the graft interface of the autograft ( Lycopersicon esculentum) is employed as a simple device for detecting the succes of graft unions, the accompanied structural changes at the graft junction being checked by light and electron microscopy for reference. Electrical resistance for the first 2 – 3 days increases rapidly in step with the formation and thickening of the isolation layer. In the next 3 – 8 days electrical resistance decreases steadily, as the isolation layer ruptures and disappears during callus proliferation and interdigitation. Secondary formation of plasmodesmata takes place between contacting cells of stock and scion, and differentiation of vascular elements appears in callus. Henceforth, resistance begins to drop to the level of the intact stem, which seems to indicate that symplastic connection and vascular unification have been completed. In contrast to tomato autografts, resistance measurements made across an incompatible heterograft ( Amaranthus tricolor/Lycopersicon esculentum) exhibit a steady increase with the establishment of the isolation layer, which remains unruptured. The results are discussed in relation to graft compatibility/incompatibility and the possibility of application in commercial practice.