The main purpose of the present study was to characterize the tissue and releasable molecular forms of somatostatin-28[1-12]-like immunoreactivity (S-28[1-12]LI) in rat median eminence (ME) fragments and to compare them with somatostatin-14-like immunoreactivity (S-14 LI) forms. Acetic acid extracts of ME were fractionated on Sephadex G-50 columns (in 6 M urea). The column eluate was monitored for S-28[1-12] LI by RIA with antibody R21 which detects S-28[1-12], S-28, and higher molecular weight forms of S-28[1-12] LI, but not S-14. The S-14 LI RIA utilized recognizes S-14, S-28, and prosomatostatin (pro-S). Rat ME contained 221 +/- 25 pmol S-14 LI/mg protein and 407 +/- 51 pmol S-28[1-12] LI/mg protein. By gel filtration S-14 LI was resolved into three peaks corresponding to S-14, S-28, and a higher mol wt form (14,000) corresponding to pro-S. S-28[1-12] LI consisted of at least five forms corresponding to pro-S, S-28, S-28[1-12], a form which represented pro-S without the S-14 sequence, and a form slightly smaller than S-28[1-12]. Pools of 20 ME incubated in 56 mM K+ solution showed 4.6-fold Ca++-dependent release of S-14 LI and 4-fold release of S-28[1-12] LI. Gel chromatographic analysis of the released material showed all three tissue S-14 LI forms and each of the tissue S-28[1-12] LI forms. HPLC analysis and RIAs further confirmed the release of S-14, S-28, S-28[1-12], and the S-28[1-12] LI form smaller than S-28[1-12]. These data suggest the presence of at least six molecular forms of somatostatin in ME. The release of this large number of peptides, presumably from mature secretory granules in ME in response to depolarization, suggests that they are products of the normal posttranslational processing of pro-S.