Growth hormone (GH) produced mainly by pituitary gland plays a key role in growth regulation, cell differentiation and metabolism control. Accumulating evidence in mammals indicates that it is also involved in the regulation of reproductive functions through endocrine, autocrine or paracrine pathways. In recent years, research has been undertaken tending towards the explanation the role of GH in local regulation of development and functions of the ovary and oviduct in birds. Protein and mRNA expression of GH and its receptors (GHR) was found in individual compartments of the chicken ovary. GH-immunoreactivity was more intense in the granulosa layer than in the theca layer. Moreover, secretion of GH by the granulosa cells of the largest preovulatory ovarian follicles was revealed. These observations suggest that avian ovary is an extrapituitary site of GH synthesis and GH-responsive organ. Subsequent studies have shown that the role of GH in the ovary is associated with the regulation of fundamental processes occurring in it, such as steroidogenesis, proliferation and apoptosis. It was observed that recombinant chicken GH (cGH) injected during sexual maturation caused increase in progesterone content in the chicken ovary just before and after maturation as well as increase in estradiol content before maturation. In vitro experiments revealed that cGH directly stimulated estradiol secretion by chicken prehierarchical ovarian follicles whereas inhibited release of estradiol and elevated secretion of progesterone by yellow hierarchical follicles. Farther, cGH lowered LH-stimulated secretion of estradiol by the theca layer of the three largest preovulatory follicles. Locally produced GH was able to stimulate the synthesis of progesterone by the granulosa cells of the largest follicle, which was blocked by cGH antibody. Up-regulation of cytochrome P-450scc mRNA expression (a rate-limiting enzyme in progesterone synthesis) was also shown in hen ovarian follicles. Injections of chickens with cGH from 10 weeks of age until maturation resulted in increased weight of the ovary, number of ovarian follicles and number of proliferating cells in the ovarian stroma and prehierarchical follicles. These results support earlier suggestion that GH is involved in regulation of proliferation process since injections of ovine GH to the laying hens increased the number of small follicles in the ovary. Concomitantly, cGH administration significantly diminished apoptotic cell number in the ovarian stroma and white follicles during puberty. Most recent results indicate that the avian oviduct, like the ovary, is an extrapituitary site of GH production and a target organ for GH. The expression of GH mRNA and protein has been found in oviductal segments (differing in structure and functions). Similarly mRNA expression and protein localization of GHR have been demonstrated in the chicken oviduct. Expression of GHR was lower in the infundibulum than in the magnum, isthmus and shell gland. This may indicate that GH participates in the regulation of egg component synthesis in these parts. In addition, GHR immunoreactivity was particularly present in the mucosa lining the oviduct, what further suggests involvement of GH in the production of egg constituents. Exogenous GH treatment to chickens resulted in an increase in mRNA expression of ovalbumin, major egg-white protein synthesized in the magnum, and ovocalyxins 32 and 36 (eggshell matrix proteins) produced in the shell gland as well as increase in the eggshell thickness. The action of GH in the avian oviduct appears to be also related to the regulation of apoptotic cell death, since injections of cGH for several weeks during puberty caused inhibition of cell apoptosis as identified by TUNEL assay, and reduced expression and activity of selected caspases, markers of apoptosis, in the magnum of chicken oviduct. In summary, the results so far clearly indicate that GH is involved in general mechanism responsible for activation and functioning of avian ovary and oviduct. The action of GH may reflect the effect of pituitary GH, but evidence that GH is produced in the reproductive system and parallel localized with its receptors strongly suggests that GH may also act as paracrine and/ or autocrine factor.