Abstract 1. The serotonergic cerebral giant cells of Lymnaea stagnalis make monosynaptic excitatory connections with identified buccal feeding motoneurones, I and 4. 2. The time course of unitary post synaptic potentials in the two cell types differs; responses in the 4-cell show longer latency and duration. 3. Post synaptic responses in 1- and 4-cells are due to increased membrane conductance, and have reversal potentials of −35 to −45mV. 4. Responses of 1- and 4-cells to localised application of 5-HT have similar time courses, and reversal potentials of −30 to −45mV. 5. Post synaptic responses in the two cell types can be distinguished pharmacologically.