The biological activity of a new analogue of oxytocin in which the tyrosyl group is replaced by phenylalanyl.

A new synthetic analogue of oxytocin with a phenylalanyl group instead of a tyrosyl group has been investigated. The most prominent biological properties of this peptide were its oxytocic activity on the rat or cat uterus in vivo and on milk-ejection pressure in the rabbit. Another remarkable feature was the striking difference in pressor activity measured in spinal cats and in anaesthetized rats. The presence of the phenylalanyl group in the molecule in the above-mentioned position did not increase the antidiuretic potency in unanaesthetized rats. The authors propose a nomenclature for the peptides related to oxytocin and vasopressin.