An attempt was made to determine the possibility of using pyrrole formation in hair as a biological marker for exposure to industrial neurotoxicants. Male Sprague-Dawley-rats were injected intraperitoneally with 50mg/kg 2,5-hexanedione (110134) (2,5-HD) daily for 45 days. Urine samples were collected at 16 and 24 hours (hr) after treatment, analyzed for pyrrole like substances by spectrophotometry, and compared to individual samples collected 3 days before injection. Body hair was removed from anesthetized rats at intervals and analyzed for the presence of pyrrole like substances with p-N,N-dimethylamonibenzaldehyde (DMAB). Immunofluorescent studies of rat skin utilizing polyclonal antibodies to protein bound pyrroles were also conducted. Control hair samples stained yellow after DMAB treatment, in contrast to hair from 2,5-HD treated rats, which stained reddish brown. Hair samples from rats 25 days after cessation of 2,5-HD treatment also stained reddish brown. Solubilized extracts of hair proteins from 2,5-HD treated rats yielded positive Ehrlich's test results, while control extracts gave negative results. The absorption spectra of 2,5-HD treated hair extracts demonstrated an absorbance maximum at 530 nanometers, corresponding to 2,5-dimethylpyrrole. Vibrissae taken after 1, 2 and 5 weeks of 2,5-HD treatment showed a progressive increase in positive stained area. Immunofluorescent studies of rat skin showed a significantly greater staining of hair bulbs from 2,5-HD treated rats than from controls. The average daily output of urinary pyrrole like substances of four 2,5-HD treated rats was determined for the 45 day course of treatment. In the 24hr period following 2,5-HD injection, 96% of pyrrole excretion occurred during the first 16hr. No significant changes in average daily pyrrole excretion was observed over the 45 day period. The authors conclude that hair analysis for pyrrole content is a suitable biological marker for chronic exposure to neurotoxicants that are metabolized to 2,5-HD.