Bath application of 0.5 and 2 microM acetylcholine (ACh) slowed the decay phase of miniature endplate currents (MEPC) recorded in isolated, voltage-clamped and prostigmine-treated frog sartorius muscle. Washout of ACh led to a decrease of the decay time constant of the MEPC to 72 +/- 5% (n = 5) and 51 +/- 3% (n = 6) of initial values, respectively, followed by very slow and incomplete recovery. MEPC amplitude changed slightly and recovered relatively fast. This discrepancy in the recovery rates is suggested to be due to a 'trapping' ability of desensitized receptors which can compete with the free receptors for ACh molecules and prevent repetitive binding. Thus the high affinity of desensitized receptors to ACh may partially compensate the absence of acetylcholinesterase activity.