The synaptic interactions among a group of cells in the leech C.N.S. that regulate the animal's heartbeat exhibit several remarkable features (Thompson & Stent, 1976 a, b, c). We have examined in detail the properties of the inhibitory synapse between two of these cells, the heart interneurone (HN cell) and the heart excitor motoneurone (HE cell). 1. Impulses in the presynaptic HN cell gave rise to monosynaptic i.p.s.p.s in the HE cell that were blocked by high concentrations of Mg and were reversed when the membrane potential of the post-synaptic motoneurone was hyperpolarized beyond--75 m V or when Cl was injected into the cell body. These i.p.s.p.s were chemically mediated, and involved an increase in chloride conductance. 2. In contrast to chemical synapses between sensory and motor cells in the leech C.N.S., little facilitation or depression of transmission occurred when the HN cell was stimulated at frequencies of 0.1--50 Hz. 3. Steady subthreshold depolarization of the presynaptic HN interneurone evoked a maintained hyperpolarization of the post-synaptic HE cell, indicating that currents injected into the HN cell body could spread to the terminals and cause continuous release of transmitter. 4. The size of the i.p.s.p. evoked in the HE motoneurone by an action potential in the HN interneurone varied with the resting membrane potential of the presynaptic cell. An impulse superimposed on a prolonged, subthreshold, depolarizing pulse produced a larger i.p.s.p.; conversely, prolonged hyperpolarization of the HN interneurone reduced the i.p.s.p. amplitude recorded in the HE cell. This effect was most obvious when the natural, rhythmical bursts of activity in the HN interneurone were interrupted by bathing the preparation in leech Ringer fluid containing elevated concentrations of Mg. Under these conditions a 10 mV depolarization of the HN cell increased the size of the i.p.s.p. in the HE cell approximately sixfold. Significant changes in i.p.s.p. amplitude occurred without any noticeable change in the amplitude and duration of the presynaptic action potential. With large presynaptic depolarizations, which produced the biggest i.p.s.p.s, there was some reduction in the amplitude and increase in the duration of the action potential. 5. Following a step depolarization of the presynaptic cell, the size of successive i.p.s.p.s increased with a time constant of about 1 sec. Upon repolarization the i.p.s.p.s decreased in amplitude to the original level. 6. stimulation of one HN cell also gives rise to an i.p.s.p. in its contralateral homologue (Thompson & Stent, 1976c). Trains of i.p.s.p.s produced in this way hyperpolarized at HN cell to such an extent that the size of the synaptic potential it evoked in an HE cell was reduced. 7. Thus, an HN interneurone inhibitis transmission between the contralateral HN and HE cells presynapitcally in addition to inhibiting directly the ipsilateral HE motoneurone.