AC-178335 is a pure somatostatin (SRIF) antagonist with an affinity constant (Ki) of 172 +/- 12 nM. AC-178335 blocks SRIF inhibition of adenylate cyclase in vitro (IC50 = 5.1 +/- 1.4 microM). It induces GH release when given alone (50 micrograms intravenously) to anesthetized rats with or without pretreatment with a long-acting SRIF agonist.