Nine active neurohypophyseal principles have been isolated and identified among 
the vertebrates. Arginine-vasotocin is the most ubiquitous, occurring in 
pituitary glands from representatives of all the major vertebrate groups. There 
is much more variation in structure among the principles that resemble oxytocin. 
The manner in which these evolved remains unclear. Arginine-vasotocin stimulates 
smooth muscles from a wide variety of vertebrate species. It can stimulate 
contraction of oviducts from many jawed fishes and tetrapods. The oxytocin-like 
peptides are usually less active in this respect. Among adult mammals 
arginine-vasotocin is replaced by arginine-vasopressin which has much less 
oxytocin activity. Thus, although arginine-vasotocin may both stimulate oviducts 
and cause water retention in nonmammalian tetrapods, oxytocic and antidiuretic 
functions can be regulated independently by oxytocin and vasopressin in mammals. 
Arginine-vasotocin elicits vasoconstrictor responses in even the most primitive 
vertebrates. These may be systemic or regional. Their distribution may determine 
whether arginine-vasotocin acts as a diuretic or an antidiuretic agent. It is 
possible that the most primitive neurohypophyseal functions were related to 
cardiovascular regulation and that the neurohypophysis acquired its 
osmoregulatory functions later in vertebrate evolution.