Pituitary gland is an internal secretion gland that controls and regulates the functioning of other glands. It contains lactotrophs, hormone prolactin (PRL) secreting cells. Estradiol (E2) is a hormone whose production is regulated by the pituitary gland and contributes to the formation of prolactinoma, which is a benign tumour of the pituitary gland. L-DOPA is commonly used as a first line of treatment for Parkinson's disease. It is a dopamine precursor that increases levels of dopamine for a short period, thereby inhibiting the action of PRL and consequently the formation of prolactinoma. Synaptotagmin 4 (SYT4) is an early response gene, which is involved in the release of vesicles. The aim was to investigate whether the increase in PRL secretion in ovariectomized animals with E2 implants is associated with changes in the expression of SYT4. The expression of SYT4 and PRL was analysed using immunohistochemistry, light microscopy and computerized image processing. In rats, which were receiving E2, irrespective of the presence of L-DOPA, more SYT4 was synthesised and less PRL, compared to adenohypophyses of animals where E2 was not present; in those animals, less SYT4 and more PRL was synthesized. Compared with the PRL quantities measured by immunohistochemistry, the serum PRL concentrations showed the opposite result.