Several analogues of L-prolyl-L-leucylglycinamide (PLG) were synthesized wherein the prolyl residue was replaced with other heterocyclic amino acid residues. Among the analogues synthesized were D-Pro-Leu-Gly-NH2 (2), less than Glu-Leu-Gly-NH2 (3), Thz-Leu-Gly-NH2 (4), Pip-Leu-Gly-NH2 (5), Aze-Leu-Gly-NH2 (6), L-delta 3,4-Pro-Leu-Gly-NH2 (7), and D-delta 3,4-Pro-Leu-Gly-NH2 (8). These analogues were tested for their ability to enhance the binding of the agonist 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene to central dopamine receptors. Analogues 2, 3, and 5-7 showed activity comparable to that of PLG, while the tripeptides 4 and 8 were found to be inactive. The results show that the N-terminal prolyl residue of PLG is not an essential requirement for this tripeptide's ability to modulate dopamine receptors.