Blood-brain barrier (BBB) transport of essential polar substrates is mediated by specific, carrier-mediated transport proteins. The BBB transport mechanisms for polar compounds with terminal guanidino functional groups (R-NHC(NH)NH(2)) are not well defined. The goal of the present work was to investigate the BBB transport mechanism(s) for terminal guanidino substrates using an in situ brain perfusion technique. Brain region radiotracer influx clearance (Cl(in)) was calculated for representative guanidino substrates, [14C]L-arginine, [14C]aminoguanidine and [14C]guanidine, in the presence or absence of excess terminal guanidino analogues. The Cl(in) for [14C]L-arginine (0.21+/-0.0094 cm(3)/min/g wet brain weight, mean+/-S.E.M., n=four rats) was significantly decreased by 1000x concentrations of unlabeled L-arginine, N(G)-methyl-L-arginine, N(G)-,N(G)-dimethyl-L-arginine and N(G)-amino-L-arginine by approximately 83% (P<0.01; n=4-5), whereas 1000x concentrations of nitro-L-arginine, aminoguanidine and guanidine were without effect. In contrast, the respective Cl(in) of [14C]aminoguanidine and [14C]guanidine (0.0085+/-0.00039 and 0.015+/-0.0015 cm(3)/min/g, n=4, respectively) were not significantly decreased by 1000x concentrations of unlabeled aminoguanidine or guanidine. The Cl(in) values for all [14C]guanidino probes were significantly greater (P<0. 05) from that of [3H]inulin, a marker of cerebrovascular blood volume. These data suggest that the hydrophilic guanidino cations aminoguanidine and guanidine penetrate the BBB by a minor diffusional process with no appreciable transport via saturable processes. In contrast, BBB penetration of L-arginine occurs via the saturable basic amino acid transporter that has specificity for amino acid analogues possessing cationic terminal guanidino groups.