In the view of the fact ethanol increases the blood-brain barrier permeability for thyroxine (T4) during a critical period of postnatal brain development in the rabbit, the influence of ethanol with or without T4 on cerebral RNA and DNA concentration has been investigated. Generally accepted measures of thyroid hormone action on developing neural tissue such as the changes in the nucleic acids, cell density and size and in protein level, dry matter and cholesterol have been used. Ethanol alone caused an increase in RNA concentration and the RNA/DNA ratio. In addition, it tended to alter the brain tissue composition in the same direction as T4 administration, i.e. characteristic for an accelerated development of CNS. The changes provoked by T4 with ethanol were more pronounced when compared to those after T4 only. It is concluded that the action of ethanol might be indirect and presumably results from a greater supply of thyroxine to the brain tissue due to increased permeability of blood-brain barrier for thyroxine.