Desensitization of the nicotinic acetylcholine (ACh) receptor of Torpedo electric organs was studied by monitoring activated [3H]perhydrohistrionicotoxin binding to the channel sites of the receptor. Receptor desensitization was produced by incubating Torpedo membranes that had been pretreated with diisopropylfluorophosphate with ACh and was quenched by the addition of AChesterase. Desensitization caused concentration- and time-dependent decrease in the binding of [3H]perhydrohistrionicotoxin to carbamylcholine-activated receptors and an increase in the apparent affinity of carbamylcholine to the receptor. d-Tubocurarine and concanavalin A antagonized the desensitizing effects of ACh. The kinetics of this desensitization showed an extremely rapid temperature- and concentration-dependent component which was complete by the earliest measurement (i.e., 6 sec) and a much slower component (T1/2 = 55 sec) which reached a plateau after 120 sec. The rate of this slow component was temperature- and concentration-independent. Recovery from desensitization was temperature dependent, but independent of the ACh concentration used for desensitization. It also showed biphasic kinetics: half the receptors recovered fast (T1/2 = 36 sec), whereas the other was slow (T1/2 = 180 sec).