Entry of adenosine, and thymidine, into erythrocytes from adult dolphins was rapid, showed saturation at higher substrate concentrations, and was strongly inhibited by low concentrations of nitrobenzylthioinosine (NBMPR). Kinetic parameters were estimated from the concentration dependence of initial rates of tracer entry at 21 degrees C, as K(m) 0.14 +/- 0.05 mM and Vmax 24.4 +/- 1.9 mumol/litre cell water/sec for zero trans entry of adenosine, and K(m) 0.96 +/- 0.21 mM and Vmax 25.4 +/- 1.7 mumol/litre cell water/sec for thymidine. Adenosine, and thymidine, entry were inhibited by both purine and pyrimidine nucleosides. Mass law analysis of a saturable component of nitrobenzylthioinosine binding to dolphin red cell membranes gave values of Bmax 65.4 +/- 1.2 pmol/mg protein, and K(d) of 1.53 +/- 0.08 nM for a single class of sites. Photo-irradiation of dolphin red cell membranes in the presence of tritiated nitrobenzylthioinosine led to radioactive labeling of polypeptides M(r) 52, 500-58,000, on SDS-PAGE.