AbstractMore than 90% of the plasma content of chlorpromazine over a concentration range from 0·008 to 15·1 μg/ml was bound to human plasma protein. Binding was affected by the pH of the aqueous medium; with few exceptions the higher values were obtained at the higher pH values. Binding was highest in some of the plasma samples from humans, and successively lower in plasma from dogs, rabbits and rats. Binding of chlorpromazine after administration of the drug to psychiatric patients, and after in vitro addition of the drug to plasma, was reversible. Variation in binding in plasma from different humans was marked; the amount bound varied from 91·0 to 99·0%. Thus the variation in the amount free was from 1·0–9·0%.