2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a considerable environmental hazard in industrial societies. Its toxic effects on animals and humans are numerous, but little is known about its neurotoxicity. We studied the neurotoxic effects of TCDD in 80 male, adult Wistar rats. The substance was dissolved in com oil and a single dose injected intraperitoneally (8.8 mug, 6.6 mug, 4.4 mug or 2.2 mug/kg). Neurophysiological examinations proved a dose-related, statistically significant slowing of sensory and motor conduction velocities. Ten months after the application of TCDD peripheral nerves showed a progressive, and proximally accentuated neuropathy. The extent of changes, however, differed remarkably between individual animals. Our data indicate that TCDD caused a toxic polyneuropathy in rats.