Abstract: The respiratory metabolism of labelled glucose by leaf tissues of Nicotiana tabacum (var. Huangmiaoyü) was studied. Discs from TMV inoculated and control leaves were cultured under sterile conditions in Hoagland solution under weak light. At stated intervals after inoculation, tissues were harvested for respiration experiments in Warburg vessels using glucose-l-C14 or glucose-R-C14 as substrates. The C1/Cr ratios as well as Qo2 were followed as function of time. The following results were obtained: 1. Rates of respiration for the TMV infected as well as for the control tissues rose to a maximum up to the 4th day of incubation. It was followed by a drop on the 6th day. The rates rose again on the 8th followed by a marked drop on the 10th day. Since this trend occurred in both types of tissues it was not related to virus infection. 2. The rates of respiration in both types of tissues are about equal up to the second day of incubation. After this period, the rates in the TMV infected tissues are lower than those in the controls. This period of lowered respiration corresponded with the onset of maximum virus multiplication in the infected tissues and was maintained up to the 10th day. 3. There is a marked and consistent decline in the C1/Cr ratios with time of incubation in both types of tissues. This is interpreted as being due to changes in physiological state of host tissues upon aging similar to those observed in potato tuber slices. 4. While the relatively small and irregular changes in the values of C1/Cr ratios at the initial and final periods of incubation are not easily analysed, the consistent though small rise in the C1/Cr ratios in the infected tissues between the second and the 8th days of incubation, at the period of maximum virus multiplication is indicative of a relative shift of the pathway of glucose dissimilation towards the HMP route upon TMV infection. 5. The phenomenon of wound respiration with its accompanying metabolic changes which has hitherto been reported only in tuberous tissues, appear to be present also in tobacco leaf tissues. The need to take into consideration of this effect in studies on host-parasite metabolic relationship is stressed.