No information is at present available on the mode of SRIF biosynthesis. Since anglerfish pancreatic islet tissue is comprised of approximately 30% D cells, we have examined this tissue for SRIF synthesis . The following known differences in amino acid composition of islet peptides were used advantageously in this study: anglerfish proinsulin: Trp-0, Ile-2, Cys-6; anglerfish glucagon: Trp-1, Ile-0, Cys-0; mammalian SRIF: Trp-1, Ile-0, Cys-2. After incubating islet tissue with [3H]tryptophan and [14C]isoleucine or [35S]cystine for various time periods, proteins were extracted in 2 M acetic acid and desalted by Bio-Gel P-2 gel filtration. P-2 void volume proteins were then subjected to P-10 gel filtration and isolated by polyacrylamide gel electrophoresis (PAGE) at alkaline pH. The predominant amount of the immumoreactive SRIF in the extracts appeared in a peak eluting just before the salt volume on P-10 filtration and migrated slowly toward the cathode during PAGE. The behavior of synthetic SRIF was identical. The anglerfish SRIF immunoreactive peptide could be labeled with Trp and Cys but not Ile during incubations longer than 1 h. The Trp- and Cys-labeled peptide could be bound on columns to which the immunoglobulin fraction of antisera to SRIF had been complexed. Cycloheximide inhibited isotope incorporation into all islet proteins. These results indicate that islet SRIF is synthesized in situ. Moreover, the immunological activity, size, and charge characteristics of anglerfish islet SRIF appear to be similar to those of mammalian hypothalamic SRIF. When islets were subjected to short pulse incubations with labeled Trp and Cys, only peptides eluting in the 7,000-13,000 dalton portion of the filtration eluate became labeled. No appreciable isotope incorporation into SRIF was observed. However, when pulse incubations were followed by incubation in the presence of cycloheximide or excess unlabeled amino acids in isotope-free medium (chase), the incorporation of Trp and Cys into SRIF increased with the length of chase, suggesting the participation of a larger precursor in SRIF synthesis.