1. A histochemical technique has been devised which demonstrates cystine or cysteine in paraffin sections.
2. This depends on the oxidation of these substances with performic acid and the demonstration of the resultant cysteic acid with a basic dye, Alcian blue 8GS, at pH 0·2.
3. The specificity of this reaction depends on excluding acidic substances already present ionized in tissues at this low pH.
4. The performic acid-Alcian blue technique selectively demonstrates material with the exact distribution of Bargmann's chrome-haematoxyphil 'neurosecretory' material in the hypothalamus and in the posterior lobe of the pituitary of man, the rat and dog.
5. This material, by reason of its content of cystine, may represent posterior pituitary hormone; severe dehydration in five rats caused the almost entire loss of this material from the posterior lobe of the pituitary.
6. The performic acid-Alcian blue reaction provides the first histochemical evidence of the hypothalamic elaboration of posterior pituitary hormone in man and rat.