A single injection of cysteamine (CSH; 2-aminoethanethiol; 300 mg/kg, sc) into male rats produced a rapid decline in immunoreactive somatostatin (IR-SRIF) levels in the hypothalamus (to 20% of preinjection values within 12 h) which persisted for several days. The levels of both somatostatin-14 (SRIF-14) and somatostatin-28 (SRIF-28) were reduced. In contrast, the levels of somatostatin-28(1-12) were unaffected. Most (80-90%) of the lost SRIF molecules (both SRIF-14 and SRIF-28) could be recovered from CSH-injected rats by subjecting hypothalamic samples to denaturing, reducing, and reoxidizing conditions. These results suggest that CSH does not deplete the hypothalamus of SRIF molecules, but, instead, alters their chemical structures, rendering them undetectable using a SRIF-14-directed RIA. CSH injection also caused a rapid and complete suppression (within 1 h) of [35S]cysteine incorporation into SRIF-14 and SRIF-28. This reduction, however, was short-lived; normal incorporation rates returned within 10 h of drug administration. CSH did not influence [35S] cysteine incorporation into acid-precipitable protein or [35S]cysteine specific activity in the hypothalamus. In addition, [35S]SRIF molecules were not recovered from hypothalami of CSH-treated rats by subjecting samples to denaturing, reducing, and then reoxidizing conditions. These findings indicate that CSH injection causes a true, but short-lived (1- to 10-h), suppression of hypothalamic SRIF-14 and SRIF-28 formation. Finally, biosynthesis studies of longer duration revealed no prolonged effects of CSH. The drug produced no changes 4, 24, or 72 h postinjection in hypothalamic levels of the prepro-SRIF mRNA. Moreover, two injections of CSH, separated by 3 days, which continuously suppressed IR-SRIF levels for almost 1 week, caused only a transient suppression of [35S]SRIF-14 and [35S]SRIF-28 synthesis after each injection. These results indicate that the SRIF biosynthetic pathway is not activated by the prolonged CSH-induced depletion of IR-SRIF stores.