The synaptic connexions that underlie three different segmental shortening reflexes have been traced by recording intracellularly from individual sensory and motor nerve cells in the C.N.S. of the leech. The fourteen sensory cells involved in these reflexes respond specifically to one of three modalities: touch, pressure, or noxious stimuli applied to the skin. All three types of sensory neurone give rise to excitatory synaptic potentials in two large motoneurones. Each of these motor cells provides excitatory innervation to the longitudinal muscle fibres of the opposite side of the segment. The mechanism of synaptic transmission is, however, different for each type of sensory cell.1. An impulse in a sensory cell that responds to touch gives rise to a short-latency depolarizing potential in the large longitudinal motoneurones by way of an electrical synapse. This junction rectifies so that excitation can spread in only one direction (from the sensory to the motor cell), whereas a hyperpolarizing potential can pass only in the opposite direction.2. The synaptic potential evoked in the motoneurone by an action potential in a sensory cell responding to noxious stimuli can be attributed to the action of a chemical transmitter agent and has different properties: the post-synaptic potential arises after a delay of about 2-4 msec, is abolished by high concentrations of Mg, and enhanced by high concentrations of Ca. Several lines of evidence show that this connexion is monosynaptic.3. The synaptic potential following an impulse in a pressure cell is produced by both chemical and electrical synaptic mechanisms. Rectification, similar to that described for the touch cell, also occurs at this electrical synapse.4. One or more impulses in any one of the fourteen mechanoreceptor cells in the ganglion can initiate impulses in the large longitudinal motoneurones to produce a shortening of the segment. The contraction is abolished by blocking impulse initiation in the motoneurones.5. The arborizations of the sensory cells and the motoneurone within the neuropile have been studied histologically after injecting a fluorescent dye. Their processes are intertwined in a highly complex manner so that the sites of the synaptic junctions cannot be determined with the resolutions so far achieved. Nevertheless, taken together the histological and the electrical results support the idea that individual cells are connected in a stereotyped pattern and operate by distinctive mechanisms.6. These findings provide a basis for studying the functional role of chemical and electrical synaptic mechanisms in these pathways.