Summary A new method for isolation and purification of vacuoles from barley mesophyll protoplasts within 1 min is described. The protoplasts were disrupted by squeezing them through the needle of a syringe. The vacuoles released were flotated through a layer of silicone oil. Photosynthetic products synthesized by the protoplasts during incubation with 14 CO 2 in the light were found to be rapidly sequestered in the vacuoles. The rate of appearance of labelled products in the vacuoles was virtually identical with the rate of CO 2 -assimilation by the protoplasts after an initial lag phase of about 15 min. This indicates that, under these conditions, the vacuoles, compartments containing expandable storage pools, served as sinks for most of the newly formed photosynthetic products once the tighter, metabolic pools located outside the vacuoles had reached the steady state. More than 85% of the products formed during a 25 min period of photosynthesis were soluble substances and about 70% of this was sucrose. The rate of appearance of labelled sucrose in the vacuoles closely resembled the rate of sucrose synthesis in the protoplasts and the same was the case for malate and citrate. After an initial lag phase, labelled glutamate, glutamine, and alanine also entered the vacuoles, while transport of serine, glycine, and aspartate was insignificant during the time span of the experiments. Sugar phosphates did not appear to be transferred into the vacuoles.