When the time course for $CO_2$ fixation and $O_2$ evolution in isolated intact spinach chloroplasts was examined, we found a prolonged lag time in the early phase of photosynthesis after heat-treatment in the dark as well as an expected time-dependent decrease in the rate during the subsequent linear phase. Because the lengthening of the lag period was generally attributed to the depletion of sugar phosphates in the chloroplasts, we tested for the prosfile involvement of Calvin cycle intermediates in the change of the lag phase by heat-treatment. When triose pro phase was added to the heated chloroplasts, the lag time was re-shortened without the rate in the linear phase heir elevated to that measured in the control. Mg-ATP or triose phosphate plus oxaloacetate (previously known as proteclive chemicals) prevented the lengthening of the lag time when added prior to heat-treatment. Quantification of some metabolites in the chloroplasts confirmed that heavy losses had occurred for triose phosphate, fructose-1,6-bisphosphate, glucose-6-phosphate, and fructose-6-phosphate. However, the level of 3-phosphoglyceric acid was increased. The presence of Mg-ATP during heat-treatment alleviated the losses of those sugar phosphates. Therefore, we conclude that the decrease in sugar phosphates in the chleroplasts, as part of the negative effect from heat-treatment, is the primary cause of the lengthened lag time during the initial phase of photosynthesis.