The elongating behavior of pak-bung hairy roots was evaluated by automatic tracing of the root tip point employing computer-aided image analysis. Though the root elongation rate in the absence of herbicides was approximately constant, the addition of 1.0 mumol/dm(3) 2,4-dichlorophenoxyacetic acid (2,4-D) or pyributicarb to the culture led to gradual deterioration of the elongation rate during measurements over a 25 h period. The saturated value of the elongation rate (R-G(sat)) was determined as a measure to predict herbicidal toxicity to the roots by formulating the time profiles of-the elongation rate. Under the conditions of. 0.2-1.0 mumol/dm(3) 2,4-D and 0.1-10 mumol/dm(3) pyributicarb, the dose-response profiles based on the R-G(sat) values overlapped closely with those based on the manually determined elongation rate in prolonged cultures for 168 h. It was concluded that the system developed could be a useful tool for the assessment of herbicidal toxicity in the hairy roots.