It was demonstrated by equilibrium dialysis that dog red cells have a greater ability to bind chlorpromazine than rat red cells. Dog plasma was shown to have a greater ability to bind chlopromazine than rat plasma although this difference was not statistically significant. In the presence of CPZ (10-3M), the 50% hemolysis time of dog red cells suspended in homologous plasma or in 0.9% buffered NaCl solution was much greater than that of rat red cells treated in the same manner. The 50% hemolysis time of dog red cells suspended in homologous plasma was considerably shortened when they were suspended in rat plasma. Conversely, an increase in the 50% hemolysis time was obtained when rat red cells were suspended in dog plasma. Several possibilities for the slower rate of CPZ-induced hemolysis of dog red cells as compared to rat has been discussed in the present report.