The preparation of tetrapeptides Boc-Trp-X-Asp-Phe-NH2 (X = Ala (7), Nle (8), Pro (9), and 3PP (10)) was accomplished by using standard solution methods. For the condensation of Boc-Trp-OH with imino nitrogens, symmetrical anhydride couplings with benzyl ester side chain protection at Asp were used. After Asp deprotection by catalytic hydrogenolysis, the final products were purified by silica gel chromatography (EtOAc/pyridine/H20/HOAc systems) and characterized by mass spectrometry, NMR, and combustion analysis. Radioligand binding assays were conducted as described previously. Binding affinities for Boc-CCK4 and 7-10 to membranes from guinea pig cortex (containing CCK-B receptors) and guinea pig pancreatic acini (containing CCK-A receptors) are shown in Table I. The data for cortical receptors indicates the dramatic enhancement in affinity for the CCK-B receptor of the 3PP analogue 10 compared with the Nle analogue 8 and, particularly, the Pro analogue 9. With regard to functional activity, compound 10 is a full agonist relative to CCK8 and Boc-CCK4 in stimulating calcium mobilization in NCI-H345 cells (expressing CCK-B/gastrin receptors). Largely by virtue of its higher affinity for the CCK-B receptor, compound 10 shows substantially improved cortical selectivity (ca. 1400-fold) relative to Boc-CCK4 (ca. 74-fold). The conformational restrictions imposed by the 3PP residue provide useful information on the bioactive conformation of CCK at this receptor; in particular, the φ and χ1 angles of 3PP are restricted to a narrow range, which should closely approximate the angles found in the same region of CCK when bound to this receptor. Herein we also report on the synthesis and biological properties of a series of compounds which antagonize the action of both platelet activating factor (PAF) and histamine. To our knowledge these compounds are the first chemical entities which combine this dual activity in one molecule. Most compounds were prepared from N-methylpiperidines via demethylation (conversion to ethyl or 2,2,2-trichloroethyl carbamate followed by hydrolysis) and acylation. SAR analysis showed acetamide 3 had optimal anti-PAF activity, while removal of the C-8 chlorine atom (X position) resulted in modest loss of both anti-PAF and antihistamine activity. In vivo studies in guinea pigs confirmed the activity of selected compounds.