Activins are growth and differentiation factors belonging to the transformi
ng growth factor-beta superfamily. They are dimeric proteins consisting of
two inhibin beta subunits. The structure of activins is highly conserved du
ring vertebrate evolution. Activins signal through type I and type II recep
tor proteins, both of which are serine/threonine kinases. Subsequently, dow
nstream signals such as Smad proteins are phosphorylated. Activins and thei
r receptors are present in many tissues of mammals and lower vertebrates wh
ere they function as autocrine and (or) paracrine regulators of a variety o
f physiological processes, including reproduction. In the hypothalamus, act
ivins are thought to stimulate the release of gonadotropin-releasing hormon
e. In the pituitary, activins increase follicle-stimulating hormone secreti
on and up-regulate gonadotropin-releasing hormone receptor expression. In t
he ovaries of vertebrates, activins are expressed predominantly in the foll
icular layer of the oocyte where they regulate processes such as folliculog
enesis, steroid hormone production, and oocyte maturation. During pregnancy
, activin-A is also involved in the regulation of placental functions. This
review provides a brief overview of activins and their receptors, includin
g their structures, expression, and functions in the female reproductive ax
is as well as in the placenta. Special effort is made to compare activins a
nd their receptors in different vertebrates.