Hairy roots of pak-bung turned green when cultured under continuous light exposure, and maintained their branched root morphology with the formation of chlorophyll and the development of a thylakoid membrane system in the cells. Green hairy roots cultivated under light exhibited increased root growth and enhanced activities of superoxide dismutase (SOD) and peroxidase (POD). A light intensity of 11.1 W/m2 using white fluorescent lamps was found to support good root proliferation and relatively high activities of SOD and POD. In cultures of green hairy roots at a light intensity of 11.1 W/m2, root growth, total SOD activity and total POD activity obtained on culture day 21 were 8.1 g-dry root weight/dm3, 18 x 10(4) U/dm3 and 40 x 10(3) U/dm3, respectively. In dark culture, the corresponding values obtained on culture day 21 were 3.8 g-dry root weight/dm3, 3.0 x 10(4) U/dm3 and 9.5 X 10(3) U/dm3, respectively. It is concluded that exposure of the green hairy root culture to light caused an increased number of growing points at the root tip meristems due to the frequent branching of lateral roots.