In the photoautotrophic cultures of pak-bung hairy roots, the elongation rate of apical meristems (growing points, GPs) was examined using the root segments with varied lengths located under incident light intensities of I = 1.0, 3.3 and 11 W/m(2). It was found that there were minimum root lengths for the elongation of GPs, the values of which were 5.0 x 10(-2), 4.0 x 10(-2) and 2.0 x 10(-2) m at I = 1.0, 3.3 and 11 W/m(2), respectively. Concerning the root segments with lengths more than these minimum values, the elongation rates of GPs increased with increasing root length and light intensity under the examined conditions. It was recognized that the addition of photosynthesis inhibitor, 3-(3,4-dichlorophenl)-1,1-dimethylurea (5.0 x 10(-4) mol/m(3)), to the medium caused the reduction in the elongation rate of GI's and the enlargement in minimum root length for the elongation. Carbon balance in the hairy roots was calculated from the CO2 fixation rate considering the distribution of chlorophyll content along with root length and carbon amount to meet the increase in root mass through the elongation. As a result, a minimum carbon requirement was estimated to be approximately 9.4 x 10(-2) mol-carbon/(h . kg-DW) for the elongation of the hairy roots irrespective of the examined culture conditions.