SUMMARYSomatostatin-like immunoreactivity (SLI) from dog and rat plasma eluted from Biogel P-6 columns as three distinct peaks. A large-molecular-weight peak was present in the void volume of the column, an intermediate-sized peak (SLI 28) coeluted with synthetic somatostatin 28 (S-28), and a small-molecular-weight peak (SLI 14 ) coeluted with SRIF. Material from the SLI 28 peak diluted in parallel to the S-28 standard in the radioimmunoassay and behaved identically to S-28 on high pressure liquid chromatography (HPLC). Levels of SLI 28 in the portal vein were consistently greater than the simultaneously measured peripheral levels (portal peripheral ratio 2.2 ± 0.2). Venous samples drawn from multiple sites suggested that SLI 28 is secreted by the duodenum and/or pancreas and the intestine. This data is consistent with the possibility that S-28 is a hormone distinct from SRIF. DIABETES 37: 474-477, May 1982.