The charge of the agmatine analogues AO-Agm [N-(3-aminooxypropyl)guanidine], GAPA [N-(3-aminopropoxy)guanidine] and NGPG [N-(3-guanidinopropoxy)guanidine] is deficient as compared with that of agmatine and they are thus able to inhibit agmatine transport in liver mitochondria. The presence of the guanidine group is essential for an optimal effect, since AO-Agm and NGPG display competitive inhibition, whereas that of GAPA is non-competitive. NGPG is the most effective inhibitor (Ki=0.86 mM). The sequence in the inhibitory efficacy is not directly dependent on the degree of protonation of the molecules; in fact NGPG has almost the same charge as GAPA. When the importance of the guanidine group for agmatine uptake is taken into account, this observation suggests that the agmatine transporter is a single-binding, centre-gated pore rather than a channel.