The effects of biotin deficiency and supplementation upon saturated fatty acids in serum, liver, cerebrum, and cerebellum of rats were investigated. Serum total fatty acids were reduced in deficient animals to 29% of normal. The percentage composition of some odd-chain fatty acids was increased in the serum and liver of deficient rats. The relative composition of most saturated fatty acids with carbon-chain lengths greater than or equal to 22 were increased in serum and liver of deficient rats. In contrast, there were no differences in the brain saturated fatty acids of deficient animals compared to those of normal and supplemented animals. These results may indicate that alterations in saturated fatty acids do not play a major role in the neurologic abnormalities in human biotin or inherited multiple-carboxylase deficiencies.