Commercial hexane which caused polyneuropathy in many workers contained 10-40% of 2-methylpentane, 3-methylpentane and methylcyclopentane in addition to n-hexane. The hexacarbon compounds methyl n-butyl ketone, 2,5-hexanedione and etc. were shown to be neurotoxic like n-hexane. Therefore, 2-methylpentane, 3-methylpentane and methylcyclopentane which are also hexacarbon compounds were suspected to be neurotoxic, but their neurotoxicity had not been sufficiently investigated. The present experiment was performed to clarify their neurotoxicity by measuring the nerve conduction velocity in the rat's tail. Thirty rats were divided into five groups of 5-7 rats. n-Hexane, 2-methylpentane, 3-methylpentane and methylcyclopentane were diluted with olive oil and orally administered daily for eight weeks. The body weight, motor nerve conduction velocity, motor distal latency and mixed nerve conduction velocity were measured before administration, after two, four, six and eight weeks' administration. The n-hexane group showed a distinct impairment of the functional states of the peripheral nerve. Methylcyclopentane, 2-methylpentane and 3-methylpentane group had some significant differences in comparison with the control in the experiment, although these differences were not so distinct as those in n-hexane group. The results revealed that the neurotoxicity of the three chemicals was not so severe as that of n-hexane and were in the order of n-hexane greater than methylcyclopentane greater than or equal to 2-methylpentane divided by 3-methylpentane.