The effects of poly-L-aspartic acid on the pharmacokinetics of gentamicin were examined by using a randomized crossover trial design with the dog. When analyzed according to a three-compartment open model, poly-L-aspartic acid reduced some first-order rate equation constants (A3, lambda 1, and lambda 3), the deep peripheral compartment exit microconstant (k31), the elimination rate constant (k(el)), and the area under the concentration-time curve from 0 to 480 h (AUC0-480) (0.21-, 0.60-, 0.26-, 0.27-, 0.72-, and 0.76-fold, respectively; P < 0.05) but increased the volume of distribution at steady state (Vss), the volume of distribution calculated by the area method (V(area)), the apparent volume of the peripheral compartment (Vp), and all mean time parameters. These results suggested that poly-L-aspartic acid increased the distribution of gentamicin to or binding within the deep peripheral compartment and that poly-L-aspartic acid may have delayed gentamicin transit through the peripheral tissues. In contrast, poly-L-aspartic acid did not alter pharmacokinetic parameters relevant to the central or shallow peripheral compartments to a clinically significant extent. Although gentamicin's pharmacokinetic parameters of relevance to therapeutic drug monitoring were not directly altered, this study has provided pharmacokinetic evidence that poly-L-aspartic acid alters the peripheral distribution of gentamicin. This pharmacokinetic interaction occurred after a single intravenous dose of each drug. Therefore, this interaction should be investigated further, before polyaspartic acid can be considered for use as a clinical nephroprotectant.