The partitioning of carbon within sources and sinks, and its transport between them, is considered in relation to temperature. The characteristic accumulation of non-structural carbohydrates in both sources and sinks at low temperature is due partly to growth being more sensitive than photosynthesis to reductions in temperature, and partly to the differential sensitivity that enzymes of carbohydrate metabolism show to temperature. Translocation in the phloem is reduced by low temperature, due partly to viscosity and partly, possibly, to displacement of the contents of sieve elements; cooling slowly has much less effect than cooling rapidly. Partitioning in the whole plant has two partial processes: acquisition, the rate of import into a sink region, and allocation, the proportional distribution of assimilate between two or more competing sinks. Each of these can be affected by temperature treatment of the sink, of the source, or of the transport path. Allocation between the two halves of a barley root system held at different temperatures could not be explained by effects of temperature on metabolism, sucrose uptake or viscosity of transport in phloem.