2',3'-Cyclic nucleotide 3'-phosphohydrolase activity in the cerebrum of the congenital goiter mouse (cog/cog) is reduced in comparison with the normal heterozygote (cog/+). The weight of the cog/cog cerebrum and cerebellum were significantly less than those of the normal controls, 89.0% less for the cerebrum, and 81.1% less for the cerebellum. However, no differences were observed with regard to DNA and RNA content and the RNA/DNA ratio. The results of this study indicate that hypomyelination in the congenital goiter mouse is restricted to the cerebrum, and is not related to arrested glial proliferation.