The investigation of naturally occurring neurohypophysial hormones demands methods of separation applicable to microgram quantities. The first effective method developed was the paper chromatographic procedure of Heller & Lederis (1958). Thin-layer chromatography is quicker, its reliability is much less affected by environmental factors and it has excellent resolving power. It has therefore replaced paper chromatography for many purposes and an attempt was therefore made to use it for the separation of neurohypophysial peptides. The adsorbent was cellulose powder (Macherey, Nagel and Co., No. MN 300, free of calcium sulphate). The plates were prepared by mixing 1·5 g. dry cellulose powder with 9 ml. of water, until a smooth slurry resulted. The slurry was spread on the glass plate by means of a Perspex plate, 10·2 × 7·5 cm. in size, with a recess of 8·2 cm. × 0·5 mm. machined in one margin. The plate was dried at 105° for