Glutathionylspermidine synthetase (GspS) is an essential enzyme in the biosynthesis of trypanothione and is an attractive target for the design of selective anti-parasitic drugs. We synthesised a series of analogues of glutathione (L-γ-Glu-L-Leu-X) where the glycine moiety has been substituted for other amino acids. These peptides were evaluated as substrates and inhibitors of GspS. Compounds with basic side chains such as diaminopropionic acid were found to be good inhibitors (Ki: 7.2 μM). Substitution of the glycine part abolished the GspS substrate properties of the tripeptide.