The presence of a »Somatotrophin-Releasing Factor« (SRF) in acetic-acid extracts obtained from dog hypothalami was demonstrated by three kinds of assays: an »in vitro« test derived from that of Saffran & Schally (1955 b) for the measurement of CRF; a modification of the tibia test, performed on intact immature female rats; and measurement of the weight gain of immature female rats during prolonged treatment. The extracts were active only on intact animals, and not directly on hypophysectomized ones. The activity was not observed in other parts of the brain than the hypothalamus region. Crude post-pituitary extracts were contaminated with SRF, although the synthetic neuro-hormones were not »somatotrophin-releasing« substances. SRF is therefore different from vasopressin.

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