Somatostatin-28 is an hormonally active peptide secreted into hypophysial portal vessel blood.

Somatostatin 14 and 28 (SS-14 and SS-28) have been measured in hypophysial portal blood of male Wistar rats. The concentration and total amount of SS released into portal blood were increased 6- to 7-fold by electrical stimulation of the median eminence. On HPLC, only two peaks of immunoreactivity were detected in extracts of portal plasma and these peaks corresponded with SS-14 and SS-28, respectively. The amount of SS-28 in portal plasma was, on a molar basis, similar or greater than that of SS-14 and this together with other evidence suggests that SS-28 is an hormonally active peptide.