Boc-[(N-Me) Nle31, 1Nal-NH2331CCK,(2)(KI= 2.8 nM), BOC-[P~~~~,~ N~~-NH~~~ ICC& (15)(KI= 14 nM), and Boc-[P~~~~,~ N~~-N (CH~)~ WCK,(17)(KI= 39 nM) displayed good affinities for brain CCK-B receptors and had good selectivity ratios. These pseudopeptides, in which the presence of unnatural and hydrophobic residues is expected to improve their penetration of the central nervous system, were shown to be very resistant to brain ...