The binding and uptake of the bee venom toxin apamin, by guinea-pig and rat liver were studied. Guinea-pig liver plasma membranes contain inhibitable, high-affinity binding sites for [125I]monoiodoapamin: Kd = 12.6 +/- 0.8 pM (SE); Bmax = 4.2 +/- 0.2 fmol/mg protein. No binding sites for [125I]monoiodoapamin on rat liver plasma membranes were detected in agreement with the absence of a physiological response to the toxin by rat hepatocytes. [125I]Monoiodoapamin, injected into the portal vein of guinea-pigs, was recovered in an undegraded form in a liver endosome fraction. The uptake of [125I]monoiodoapamin by rat livers was less than 4% of that taken up by guinea-pig livers and there was little evidence of radiolabelled toxin appearing in isolated rat endocytic vesicles. Inhibitable, high-affinity binding sites for [125I]monoiodoapamin were also identified on isolated guinea-pig liver endosomal membranes; Kd = 10.6 +/- 3.3 pM; Bmax = 2.5 +/- 0.6 fmol/mg protein. No inhibitable apamin binding sites were detected on rat endosomal membranes. Plasma membranes and endosomal membranes isolated from guinea-pig liver showed a similar spectrum of polypeptides to that previously reported for plasma membranes and endosomal membranes isolated from rat liver. The enzymatic composition of guinea-pig endosomes was also similar to that previously reported for rat endosomes. The results indicate that apamin was internalised by receptor-mediated endocytosis by guinea-pig liver cells in an analogous manner to that already shown for a variety of endogenous ligands.