The influence of atmospheric humidity and soil moisture on the translocation of foliar applied sucrose-C^14 was studied in the sweet potato plant, Ipomoea Batatus Lam. Sweet potato plants to which sucrose-C^14 was applied on a single leaf were held in dark or light conditions for 10 hours at 70 % and 100 % air humidity respectively. The C^14 accumulation rates in root were much higher at 70 % air humidity than at 100 % air humidity in dark and light conditions. When roots of a sweet potato plant were divided into two groups and each of the halves was exposed to different soil moisture for 16 hours in darkness, the translocation and storage of C^14 was better in low soil moisture (40 %) than in high soil moisture (100 %).