The distribution of ofloxacin (OFLX) along a single hair shaft was analyzed in detail for use as an index of hair growth and as a time marker for drug analysis in hair. A single hair obtained from each of seven subjects, who had taken OFLX for 1-4 days (total of 200-1,200 mg) 2.7-5.3 months before hair sampling, was cut into 1-cm-long portions successively from its scalp end. OFLX in each hair portion was measured by high-performance liquid chromatography with a fluorescence detector, and the distance from the scalp end of the hair portion containing OFLX was determined. Then the other 2-cm long segment of hair, which had the above-determined distance at its middle, was cut successively into 2-mm-long pieces and OFLX was determined in each piece. This procedure was repeated in a total of three to four hair strands collected from one subject. OFLX was observed to distribute only in one to three consecutive 2-mm-long pieces of hair, showing no large diffusion of OFLX along the hair shaft with time. Therefore, OFLX distribution may serve as a time marker for analyzing other drugs in hair. Hair growth rate could be thus estimated and ranged from 0.99 to 1.27 cm/month (1.12 +/- 0.11 cm/month, mean +/- SD) among individuals. The intraindividual variability of hair growth rate was 4.8-18.1% (10.3 +/- 5.1%) as coefficient of variation.