The pattern of translocation and distribution of C14 labeled photo-assimilates in watermelon and tomato grown in the greenhouse and field was characterized. Each of the mature leaves of the plant at active fruit development stage was exposed to 14CO2 (20 μCi radio activity) for 40 min and the leaves, stems, fruit, and roots were harvested 3, 6, 9, or 12 hours after treatment. One half of the plants were grown under natural light and the other half in the dark during the experimental period. The activity of C14 in the dry tissues of the leaves, stems, fruits, and roots was determined, using a liquid scintillation analyzer. Both watermelon and tomato plants grown in the greenhouse and field contained C14 in all tissue types 3 hr after treatment, regardless of exposure to light or dark during the experimental period. Watermelon and tomato, respectively, transferred 22% to 61% and 9% to 26% C14 from the source leaf in 3 hours. Fruit tissues served as the strongest sink, with the highest percentages of C14 transfer in watermelon (99%) and tomato (90%) in plants grown in the field. The rate of C14 translocation was highest when plants were kept in the dark after 14CO2 feeding. In general, total translocation of C14 compounds from the source leaf was higher in watermelon than in tomato plants. For both watermelon and tomato, most field-grown plants showed a higher rate of C14 translocation as compared to greenhouse grown plants for a given period of time.