Citrated, platelet-rich plasma from human, guinea-pig and rat blood was incubated at 37 °C with (8-14C)-adenosine at different concentrations (1, 3, 30, 100 µM) with continuous stirring. Nucleotides and nucleosides were separated by high voltage electrophoresis for 1 h in 0.05 M citrate buffer pH 4.5. The spots were eluted and radioactivity was counted. Labelled adenosine was taken up by platelets within a few seconds and radioactivity increased with time of incubation. Guinea-pig platelets incorporated labelled adenosine more slowly than the platelets of other species: after 3 min of incubation with 1 µM (8-14C)-adenosine, 43% of the total radioactivity was found, whereas human and rat platelets incorporated 73 and 72% of the total radioactivity, respectively. The kinetics of incorporation and of nucleotide formation by the enzyme adenosine kinase indicated that the Km was 72.46 µM and the Vmax was 0.85 µM/min in guinea-pig platelets; the Km was 5.06 µM and the Vmax was 00.14 µM/min in human platelets; the Km was 13.72 µM and the Vmax was 0.74 µM/min in rat platelets. The kinetics of deamination and nucleoside formation by adenosine deaminase indicated that the Km was 28.21 µM and the Vmax was 3.22 µM/mm in guinea-pig platelets; the Km was 26.40 µM and the Vmax was 1.41 µM/min in human platelets, but the Km was 60.25 µM and the Vmax was 1.49 µM/min in rat platelets. It was concluded that there is no correlation between the inhibitory activity of adenosine on ADP-induced aggregation in the various species and the rate of labelled adenosine incorporation