Abstract: The processes of tobacco (Nicotiana tobaccum) callus initiation,development and redifferentiation,and the associated changes of respiratory metabolism pattern were studied. Dedifferentiation was found to be completed by phases. The callus cells formed during the initiation phase (within 2 days) could not get rid of the influence of the programmed pattern of formation from the mother leaf explants,until they had passed through the development phase and entered the maturation phase (after day 8).Otherwise,the callus cells produced only gravitropic leaf-like bodies on bud differentiating medium. At the crucial development phase,a climacteric-like respiration peak composed of the enhanced SHAM sensitive and Na 3PO 4 sensitive pathways took place,while RQ fell to a low value of less than 1 0. At the same time,TCA cycle contribution to the rate of CO 2 release decreased drastically,causing its partial respiratory quotient (PRQ) also to go down to less than 1 0. During bud primordium formation on differentiation medium,a similar respiratory pattern change was observed. The physiological roles of the metabolism changes were discussed.