The nitrogen-sparing effect of methionine, methionine sulphoxide, homocystine, cystine and choline was studied in rats by determining dialy N excretions during 5 d after changing from a high-protein diet to a protein-free diet, L-glutamic acid was used as a negative control. 2. L-, D- and DL-methionine were equally active in sparing N. L-methionine sulphoxide, DL-homocystine and L-cystine were as active as L-methionine. D-methionine sulphoxide was slightly less active than L-methionine sulphoxide. Choline hydrogen tartrate was not different from the negative control. 3. It is concluded that in short-term experiments cystine is the key substance in the N-sparing effect.