The cellular and subcellular organization of thyroid follicular cells in peripubertal (6-month-old) male pigs treated with estradiol-dipropionate (Oe) plus progesterone (Pr) in combination on the first postnatal day was studied. A stereological method was used for morphometric determinations of the relative volume densities of the follicular epithelium, colloid and interstitium, and for establishing the epithelial height and index of activation rate. Statistically significant differences of the examined parameters between the control and Oe+Pr -treated groups were determined by Student's t-test. The subcellular organization of thyroid follicular cells was studied using transmission electron microscopy (TEM). When compared with the control group, in the Oe+Pr -treated pigs, thyroid follicles possessed a noticeably higher follicular epithelium when compared with the control animals. The observed changes were quantified and comparison between the experimental groups showed that the height, volume density of follicular epithelium, and index of activation rate were significantly (P < 0.05) increased, whereas the relative volume density of the colloid was significantly (P < 0.05) decreased. At the subcellular level the thyroid follicular cells of Oe+Pr -treated pigs were characterized by increased number of polysomes and dense bodies and extensive endoplasmatic reticulum. It was concluded that a single neonatal treatment with female gonadal steroids exerted a prolonged effect on the pig's thyroid, characterized by increased biosynthesis and reabsorption of the colloid by the follicular cells.