The effects of methyl n-butyl ketone (MBK) on nervous system function and operant behavior were investigated in monkeys and rats. Mean MBK exposure levels approximated 100 and 1000 ppm and lasted up to 10 months. Both exposures were 6 hours/day, 5 days/week. Results showed that the 1000 ppm exposure effected 1) reduced motor conduction velocities in ulnar and sciatic-tibial nerves, 2) decreased the amplitude of evoked muscle action potentials, 3) lengthened implicit time of visual evoked potentials, 4) impaired operant behavioral performance, and 5) reduced body weight. Reductions in nerve conduction velocities and evoked muscle action potentials were found at 100 ppm. Recovery, as measured by sciatic-tibial nerve conduction velocity, was found to occur 6 months and 2 months after termination of the 1000 ppm and 100 ppm MBK exposures, respectively.