Phenolic compounds extracted from the graft union of heterograft trees (Prunus persica/P. tomentosa) were analyzed by histochemistry and HPLC methods and compared with those on homograft trees (P. persica/P. persica). A DMACA (p-dimethylamino-cinnamaldehyde) staining method showed that at early stages after budding the phenols were localized mainly in the callus cells, in the new xylem and in the cortical tissues of the heterograft combination and persisted even 6 months after budding. These phenolic compounds frequently surrounded necrotic tissues which seemingly impeded the union between the scion and rootstock. From the HPLC analysis, four phenolic compounds were found as major components in both rootstocks, but two of them were not detected in the phloem of dwarfing P. tomentosa. The chemical nature of these peaks was unknown, except one was identified as catechin ; however, based on their UV spectrum, behavior in TLC and retention time, our assignments are tentative. In general, the phenolic content in the phloem tissues was higher in homograft than in heterograft trees, particularly peaks # 1 and # 3, which we suppose are prunin and naringenin respectively. However, in the xylem tissues on heterograft trees the phenolic concentration was much smaller and the patterns from the stock to the scion were different from those observed on homograft trees. Peak # 1, prunin and Peak # 2 accumulated in the phloem above the heterograft union ; whereas in the xylem, Peak # 1, prunin, and Peak # 3, naringenin, were present at low amounts. Catechin, a low molecular weight flavan occurred in similar amounts in the xylem of both combinations, accumulating at the graft union, whereas the phloem, of the homograft contained a larger amount of catechin.