The Mauthner axon contacts two principal neurons within the brainstem: the cranial relay neuron and the follower neuron. The cranial relay neuron is excited through axoaxonic contacts by both Mauthner axons, while the follower neuron is excited by the Mauthner axon ipsilateral to its soma and inhibited (through an interposed neuron) by the contralateral Mauthner axon. We have investigated the properties of these two Mauthner axon target synapses with simultaneous pre and postsynaptic recordings in order to determine if transmission can be described in terms of the quantal hypothesis and to determine what quantal parameters change with alterations in the frequency of Mauthner cell activation. Small depolarizing postsynaptic potentials, recorded in the cranial relay neurons, increase their frequency of occurrence when depolarizing currents are applied to the Mauthner axon, without changing their mean amplitude, indicating that they arise from the Mauthner axon terminal and represent quantal units of the evoked cranial relay neuron excitatory postsynaptic potentials. Similar small, spontaneous potentials can also be recorded from the follower neurons. Amplitude histograms of evoked postsynaptic potentials (in both cranial relay neurons and follower neurons) can be fit by binomial models based on the amplitude and variance of the spontaneous potentials, suggesting that the quantal hypothesis is a valid descriptor of synaptic transmission at these contacts. Quantal content ("m"), the number of releasable quanta ("n"), and the probability of release ("p") are relatively high for both the cranial relay neurons and follower neurons. Increasing the frequency of Mauthner axon stimulation results in a decrement in the postsynaptic potential amplitude in both target cells. There is no change in the amplitude of the small, spontaneous potentials (measured during the stimulation period) with stimulus frequency, indicating a decrease in quantal content, but not quantal size. The change in quantal content is explained in binomial terms by a decrease in the number of releasable quanta, but not the probability of release in both the cranial relay neurons and the follower neurons. These findings suggest that the supply of transmitter available to be released decreases with increasing stimulus frequency without affecting the nerve terminal's ability to release transmitter (at these stimulus frequencies). Synaptic vesicles, observed in clusters in the terminals at the ultrastructural level, may be a physical correlate of the binomial parameter ("n"). It is predicted that the decrease in the binomial parameter ("n") with increasing frequency of stimulation is accompanied by a decrease in the number of active zones occupied by vesicles.(ABSTRACT TRUNCATED AT 400 WORDS)