The penetration of ornithine and citrulline into liver slices.

2. One method is based on condensation with ethyl acetoacetate in 2N-NaOH to form a product which gives a red colour with Ehrlich's reagent (p-dimethylaminobenzaldehyde). 3. The other method is a modification of the Jaff6 picrate reaction and depends on the reaction of an alkaline picrate complex of ALA with HCI to give an orange-red colour. 4. The effects of varying different factors in the determination, specificity and application to biological materials are described for both methods. I wish to thank Professor A. Neuberger, F.R.S., for his valuable advice and interest in this work, and for the hospitality of his laboratory; Dr J. J. Scott for providing many of the compounds tested in these assays; Mr B. L. Wright, Mr F. Grover andMr 0. Hill for technical assistance.