When bovine IgG, haemoglobin, or alpha -gliadin extracted from wheat, labelled with radio-iodine, are fed by stomach tube to suckling or adult rats, significant amounts of protein-bound radioactivity are found in extracts of whole brain. These amounts are greatly in excess of what can be accounted for by blood contamination: the specific activity of the brain is of the order of that of other tissues of the body, suggesting that it is permeated with protein derivatives of the fed material. This is borne out by differential centrifugation of brain macerates which shows substantial protein-bound radioactivity in the cell sap fraction. Ultracentrifugation studies reveal the presence of high molecular mass material in the cell sap, some of greater sedimentation velocity than the original protein fed, suggesting complexing of derivatives of this with native components of the brain. This is confirmed by gel filtration studies. Immunological studies on the brain extract show that a high proportion of the protein-bound radioactivity of the brain retains the ability to be precipitated by specific immune serum, when alpha -gliadin or bovine IgG has been fed.