Histochemical studies on roots of sweet potato at different stages of development revealed the presence of guaiacol oxidase and 3,3-diaminobenzidine (DAB) oxidase activities in all cells except for xylem vessels. In roots at primary and secondary stages of development similar activities were observed with highest activities in the endodermal and companion cells. In primary roots high activities were also observed in the hypodermal cells. In roots showing secondary growth cells of the periderm, which developed from the hypodermis, as well as xylem tracheids continued to show high activities. Syringaldazine oxidase showed a lag phase with weak activity in the cortical cells and high activities in all cells undergoing lignification in roots at all stages of development. Biochemical assays of guaiacol oxidase indicated high activities in both the soluble cell free (CF) and the solubilized ionically wall bound (I) enzyme fractions with the I fraction consistently showing higher activities in all stages of tuberizing roots. Qualitative tests showed that syringaldazine oxidase activity was restricted to the CF fraction. Both cofactor dependent and independent indole acetic acid (IAA) oxidase were assayed with only the I fraction in all stages of tuberizing roots showing cofactor dependent IAA oxidase activity. The implications of these results are discussed in relation to the anatomical changes and physiological processes taking place in the developing tuberous root.