The uptake of quinidine by washed human red blood cells from isotonic buffer solution (pH 7.4) occurred rapidly and was proportional to the concentration of drug in buffer. A constant red cell/buffer partition ratio of 4.16+/-0.15 s.e. mean was found. 2. Uptake from buffer solution was not affected by temperature or ouabain or by gassing with nitrogen or carbon monoxide and there was no evidence of saturability. Drug in red blood cells was associated largely with the cell contents (94.4+/-1.5% s.e. mean) following partition. 3 Plasma reduced the uptake of quinidine so that a red cell/plasma partition ratio of 0.82+/-0.09 s.e. mean was found. 4 Alteration of plasma binding by dilution of plasma with buffer showed that uptake was proportional to free drug concentration. 5 The possibility of red cell uptake of drug should be included in any considerations concerning pharmacokinetic aspects of drug action in the body.