Maximum permissible annual intakes to blood of several tritiated compounds, namely, water, folic acid, corticosteroids, sex-hormones and thymidine, have been estimated on the basis of average doses by received organs and tissues and from experiments on rats. The values range from 10 mCi for tritiated thymidine and folic acid to 1 or 2 Ci for tritiated corticosteroids. Values for corresponding 14C labelled compounds will, in general, be lower because of the higher energies of the 14C beta-particles. Dosimetry in cells is considered, particularly in relation to tritium and 14C labelled thymidine. Questions concerning the importance of inhomogeneity of absorbed energy in microscopic volumes are discussed in relation to the use of average dose as the basis for setting maximum permissible levels of dose and intakes of tritium and 14C labelled compounds.