The mechanism of interorgan variation in tissue distribution of quinidine was investigated from a viewpoint of binding characteristics to phospholipids and the composition of phospholipids in various tissues. The order of binding of quinidine to an individual standard phospholipid, expressed as a product of the association constant (K) and the number of binding sites (n), was: phosphatidyl ethanolamine (PhE) less than dipalmitoyl phosphatidyl choline (saturated PhC) less than or equal to phosphatidyl choline (unsaturated PhC) less than phosphatidyl inositol (PhI) less than phosphatidyl glycerol (PhG) less than phosphatidic acid (PhA) less than phosphatidyl serine (PhS). Thus, quinidine was found to bind preferentially to acid phospholipids such as PhS, PhA, PhG, and PhI. The greatest binding was obtained in PhS among the various phospholipids and was more than 300-fold that of neutral phospholipids such as PhC and PhE. The concentration of individual components of phospholipids in the lung, kidney, liver and heart was determined using a two dimensional thin-layer chromatography. The concentration of PhS, highly responsible for the quinidine binding to phospholipids in each tissue, was ranked in the following order: heart less than liver less kidney less than lung. The contribution of PhS to quinidine binding was more than 86% in all tissues. A good correlation between the concentration of PhS in each tissue and the Ct/Cp ratio in vivo was obtained (r = 0.984). Thus, it was concluded that the tissue distribution of quinidine in vivo depended on the composition of phospholipids in tissues and that a determinant of interorgan variation in the tissue distribution of quinidine was the concentration of PhS in the tissues.