The export of 14C from leaves of Lycopersicou esculentum (Mill.), Capsicum /rutescens (L.) and Amaranthus caudatus (L.) was followed by in vivo counting after exposure of the leaf to a 5 min pulse of 14CO~. In all instances the time course of export showed two or more exponential phases. There was an initial rapid period of export which was followed by a slower phase after about 2 h. About 12-14 h after exposure to 14C02 this second phase was superseded by an even slower phase of export which continued for more than 24 h. In tomatoes the initial phase was most rapid in plants bearing fruit which had been heated to 30~ instead of the standard 15-20~ it was slowest when the fruit were removed. In Amaranthus the rate of the initial phase was shown to be positively correlated with photosynthesis and when the latter was prevented by either darkness or the absence of CO S the rate of loss of laC was reduced. The data were used to test a model of carbon movement from a leaf which postulated the presence of two carbon pools which turned-over at different rates. The photosynthetic carbon entered the pool with the faster rate of turn-over--the 'labile' pool--and exchanged with the other, 'storage', pool. Export from the leaf was from the 'labile' pool. The results suggested that a third, longer term, storage pool should be included in the model and that the exchange between the pools should be non-linear.