The relationship between the callus formation and intracellular Ca 2+ at the early stage of development of graft unions in Ginkgo biloba L. was investigated. On the second day after grafting, part of the undamaged living cells at the graft interface of the stock and scion in the regions of the cortex and pith began to dedifferentiate or divide. 8 d after grafting, a great number of callus cells were produced from the cortex and phloem, while callus bridges linking the stock with scion formed between the callus cells of the graft partners from the cortex. A membrane Ca 2+ probe, chlortetracycline (CTC), was used to locate the intracellular Ca 2+ distribution and it was found that the cortical and pith parenchyma cells were the first to show remarkably increased CTC Ca 2+ fluorescence, suggesting that the cortical and other parenchyma cells play a leading role at the early stage of development of graft unions by earlier dedifferentiation and more rapid production of callus cells. The relationship between the callus formation and intracellular Ca 2+ at the early stage of development of graft unions in Ginkgo biloba L. was investigated. On the second day after grafting, part of the undamaged living cells at the graft interface of the stock and scion in the regions of the cortex and pith began to dedifferentiate or divide. 8 d after grafting, a great number of callus cells were produced from the cortex and phloem, while callus bridges linking the stock with scion formed between the callus cells of the graft partners from the cortex. A membrane Ca 2+ probe, chlortetracycline (CTC), was used to locate the intracellular Ca 2+ distribution and it was found that the cortical and pith parenchyma cells were the first to show remarkably increased CTC Ca 2+ fluorescence, suggests that the cortical and other parenchyma cells play a leading role at the early stage of development of graft unions by earlier dedifferentiation and more rapid producti on of callus cells.