The elongating potentials of pak-bung hairy roots were characterized in terms of ATP contents in root tip parts using the heterotrophic (HT) and photoautotrophic (PT) roots. When the HT roots with an initial ATP content in tip parts of 8.1 x 10(-5)-3.6 x 10(4) mol/m were cultivated in a sucrose-free medium, root elongation was found to cease at an ATP content below about 2.0 x 10(-4) mol/m. In the case of the PT roots cultivated under light irradiation, the occurrence of root elongation depended on the root length from the root tip (L-0), i.e., the roots with L-0 = 2.0 x 10(-2) m did not elongate but the root with L-0 = 5.0 x 10(-2) m did. From the time profiles of root elongation and ATP contents, cell yield of elongating roots and maintenance energy on a ATP basis were estimated to be 1.1 x 10(4)-1.2 x 10(4) m/mol and 2.4 x 10(-6) mol/(h.m) for the HT roots, and 1.2 x 10(4) m/mol and 2.2 x 10(-6) mol/(h.m) for the PT roots, respectively, coinciding closely with each other under the examined conditions.