This paper gives a detailed account of techniques which can be used to measure [3H]phencyclidine binding to its receptor. The main properties of the binding component are the following: (i) It is rapidly heat-inactivated at temperatures over 50 degrees C. (ii) It is destroyed by proteases like trypsin, pronase or papain suggesting that it is of a protein nature. The receptor structure is resistant to chymotrypsin. (iii) A good correlation was found between the pharmacological activity of 30 different analogs as measured by the rotarod assay and the affinity of these different molecules for the phencyclidine receptor. (iv) Monovalent and divalent cations antagonize [3H]phencyclidine binding to its receptor. The dissociation constant is 15 mM, the same for Na+, Li+, K+, cholinium or Tris. Na+ (and other monovalent cations) and phencyclidines bind to distinct sites. The saturation of the Na+ site by Na+ modulates the affinity of phencyclidine for its receptor. Divalent cations antagonize [3H]phencyclidine binding in the absence of Na+. This antagonism is of the non-competitive type. (v) [3H]phencyclidine binding is also antagonized by histrionicotoxin and by local anaesthetics.