The two forms (DTyr8 and LTyr8) of the putative somatostatin sst2 receptor antagonist CYN 154806 (Ac-4NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-D/LTyr-NH2) were investigated on recombinant human somatostatin receptors and endogenous guinea-pig ileum receptors. In radioligand binding studies using the agonist radioligands [125I]LTT-SRIF-28, [125I][Tyr10]cortistatin-14, [125I]CGP 23996 and [125I][Tyr3]octreotide in Chinese hamster lung fibroblast (CCL39) and Chinese hamster ovary (CHO) cells expressing human somatostatin receptors (hsst1-5), CYN 154806 binds to sst2 receptors with nanomolar affinity (pKD=8.14-8.89), 40- to 4500-fold higher than for sst1, sst3 or sst4. High affinity was also demonstrated for sst5 receptors, particularly for LTyr8CYN 154806 where the sst5 affinity was higher than for sst2 receptors when using [125I]CGP 23996 and [125I][Tyr3]octreotide. Functional properties of the compounds were examined in Chinese hamster ovary (CHO) cells expressing human sst2 receptors, in (1) inhibition of forskolin-stimulated adenylate cyclase, (2) stimulation of serum response element-driven luciferase expression and (3) [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPS) binding. L- and DTyr8CYN 154806 showed full agonism at inhibition of forskolin-stimulated cAMP accumulation (pEC50=7.73 for both, Emax 104% and 78%, respectively), partial agonism at luciferase expression (pEC50=7.85 and 8.16, Emax=50% and 29%, respectively) and behaved as apparently silent antagonists at [35S]GTPS binding (no agonism observed, pKB=6.88 and 7.50, respectively). The agonist potential was confirmed in isolated guinea-pig ileum preparations via measurement of SRIF-induced inhibition of neurotransmission, where the L-isoform had marked agonism (pEC50=8.23, Emax=32%) whereas the D-isoform was apparently devoid of agonism. The present data suggest that CYN 154806 should be used with caution as an sst2 receptor antagonist tool, since it possesses intrinsic activity at sst2, and high affinity for both sst2 and sst5 receptors. The DTyr form, having lower intrinsic activity, especially in natural tissues, and greater selectivity for sst2 receptors, may be more reliable than LTyr CYN 154806.