N,N-Dimethylation of the H-Dmt-Tic-NH-CH(R)-R' series of compounds produced no significant affect on the high delta-opioid receptor affinity (K(i)=0.035-0.454 nM), but dramatically decreased that for the micro-opioid receptor. The effect of N-methylation was independent of the length of the linker (R); however, the bioactivities were affected by the chemical composition of the third aromatic group (R'): phenyl (Ph) (5'-8') elicited a greater reduction in micro-affinity (40-70-fold) compared to analogues containing 1H-benzimidazole-2-yl (Bid) (9-fold). The major consequences of N,N-dimethylation on in vitro bioactivity were: (i). a loss of delta-agonism coupled with the appearance of potent delta antagonism (4'-7') (pA(2)=8.14-9.47), while 1 exhibited only a 160-fold decreased delta agonism (1') and the delta antagonism of 8 enhanced >10-fold (pA(2)=10.62, 8'); and (ii). a consistent loss of micro-affinity resulted in enhanced delta-opioid receptor selectivity. With the exception of compound 1', the change in the hydrophobic environment at the N-terminus and formation of a tertiary amine by N,N-dimethylation in analogues of the Dmt-Tic pharmacophore produced potent delta-selective antagonists.