The action of ketamine (50 microM) and 4-aminopyridine (4-AP) (50 microM) on the frog (Hyla crepitans) neuromuscular junction using standard intracellular recording techniques was investigated. In our experiments the amplitude of evoked end plate potential was decreased by ketamine and increased by 4-AP, which is similar to the effects reported by other investigators. However, novel effects of the drugs on postsynaptic acetylcholine receptors were detected when miniature end plate potential (MEPP) amplitude and time-course were analyzed. The experiments were performed and registered in presence of 1% dimethylsulfoxide to augment frequency of MEPP's. When ketamine was added to the saline solution, MEPP amplitude control values were reduced from 226 (214, 240) microV (n = 529, median and its 95% confidence interval) to 140 (130, 151) microV (n = 143) by 50 microM ketamine. 4-AP alone also depressed MEPP amplitude to 129 (70, 152) microV (n = 91). The two drugs are partial antagonists, when they were applied together, MEPP amplitude was 180 (172, 188) microV (n = 207). The effect of ketamine on time course of MEPP's, expressed as time to peak, reduced the control values of 4.74 (4.61, 4.88) msec (n = 529) to 3.84 (3.72, 4.01) msec (n = 143) (P less than 0.001); the effect of 4-AP was to reduce the control values to 1.93 (1.79, 2.10) msec (n = 91). The time to peak after adding ketamine and 4-AP was 3.25 (3.12, 3.38) msec (n = 207), expressing an antagonistic effect of the drugs. All values are statistically distinct (P less than 0.001). Thus, our results show that a postsynaptic mechanism is responsible for a significant part of the antagonism of ketamine effects by 4-AP.