Intracellular recordings were made from identified neurons in the suboesophageal ganglionic mass of the snail, Helix aspersa. The ionic mechanisms associated with acetylcholine excitation and inhibition, dopamine excitation and inhibition, gamma-aminobutyric acid (GABA) excitation and inhibition and serotonin excitation were investigated. Acetylcholine excitation was found to involve an initial increase in sodium conductance while acetylcholine inhibition was a pure chloride event which reversed at membrane potentials more negative than the chloride equilibrium potential. Dopamine excitation appeared to involve only an increase in sodium conductance while serotonin excitation involved an increase in conductance to both sodium and calcium ions. Dopamine inhibition was associated with an increase in potassium conductance but failed to reverse at membrane potentials more negative than the potassium equilibrium potential. GABA excitation involved conductance increases to both sodium and chloride ions while GABA inhibition was a pure chloride event. An attempt was made to estimate the degree of co-operativity of the putative transmitters with their receptors using log-log and Hill plots. The slopes of the line for the log-log plots for acetylcholine excitation and inhibition were 0.88 and 1.1, respectively, suggesting the interaction of one molecule of acetylcholine with the receptor. The slope of the log-log plot for dopamine inhibition was 0.46 while that for serotonin excitation was 0.75. The Hill plots for GABA excitation and inhibition were 1.64 and 1.42, respectively, suggesting that two molecules of GABA are required for receptor activation.