It is assumed that hypothalamic somatostatin plays a role in the preovulatory phase of the oestrous cycle in sheep. The aim of the study was to investigate the processes of synthesis, storage and release of somatostatin in hypothalamic neurons, in immature female lambs, in the period approaching to puberty. Experiments were carried out on 10 prepubertal (17 weeks old) and 10 peripubertal (32 weeks old) ovary-intact lambs. Morphofunctional changes in the somatostatin neurons were assayed with immunohistochemistry and hybridisation in situ. Computer image analysis was used to determine the density of both reactions and the percentage of the area exhibiting immunohistochemical staining. These parameters express the content of immunoreactive (ir) somatostatin and expression of mRNA for prepro-somatostatin (PPS). Two populations of ir somatostatin perikarya were localized in the hypothalamus: a very large number of perikarya in the periventricular (PEV) nucleus, and single cell bodies in the arcuate (ARC) nucleus. Only it somatostatin fibres, but no perikarya were seen in the ventromedial (VM) nucleus and preoptic area. The analysis of mRNA PPS showed perikarya filled with silver grains localized in the PEV, ARC and VM, There were differences in the content of ir somatostatin and the intensity of the PPS mRNA signal between the two periods investigated. In the median eminence, the content of ir somatostatin in the terminals decreased in the peripubertal compared to the prepubertal group (P < 0.001). In the PEV, the content of it somatostatin in the perikarya and the expression of PPS mRNA decreased in the peripubertal compared to the prepubertal group (P < 0.001). In the ARC, the content of ir somatostatin in the perikarya increased (P < 0.001), but expression of PPS mRNA decreased (P < 0.001) in the peripubertal compared to the prepubertal group. There were no differences in the expression of PPS mRNA in the VM. We concluded, that the different secretory activity of the two hypothalamic populations of somatostatin neurons can be related to their different physiological functions in the prepubertal period of female lambs. (c) 2008 Elsevier B.V. All rights reserved.