EVIDENCE has recently been produced that, in the rat, oxytocin is synthesized in the paraventricular nucleus1. After small electrolytic lesions in this region a decrease in the amount of oxytocin was found in the posterior pituitary with no change in its vaso-pressin content. Robertson and Hawker2 have demonstrated the presence of a second oxytocic substance in the hypothalami of cows, oxen, cats (and kittens), rats and mice, but this substance was not present in pituitary extracts. This oxytocic substance differs from naturally occurring oxytocin in its stability towards 0.01 M thioglycollate; this procedure inactivates the hormones of the posterior pituitary3.