In long day plants such as spinach or mustard cultivated under short-day conditions (8 h oflight), the lengthening of the photoperiod results in a very large increase in glucose content which starts when the approximate floral photoperiod is reached. In the short day plant Chenopodium rubrum subjected to a transfer from continuons light to darkness, the glucose content decreases according to a mirror image ofwhat is observed in the long day plants. These results suggest that photoperiodic flower induction is characterized by a common modification of the carbohydrate metabolism and/or by a change in their compartmentation