The effects of the lectin concanavalin A (Con A), on the kinetics of desensitization of the responses of voltage clamped Achatina fulica LP5 neuron to microperfused acetylcholine (ACh) and GABA were compared. Both ACh and GABA elicited increases in chloride conductance which decayed biphasically during prolonged applications of these agonists; an initial rapid decay was followed by a later slow decay. Con A (5 micrograms/ml) accelerated both the fast and the slow decays of responses to ACh. Con A (5 micrograms/ml) also accelerated the fast decay of responses to GABA, but the slow decay was unaffected, even by 20 micrograms/ml or more of the lectin. It is suggested that, at least in the case of GABA receptor, the fast and slow decays involve distinct desensitization kinetics. The effects of Con A on the desensitization of the ACh and GABA responses were reversed by D-mannose, a competitive and specific inhibitor of Con A binding to membrane sugar residues. These results provide further evidence that receptor desensitization can be influenced by perturbing the sugar moieties associated with the subunits comprising these signalling macromolecules. The carbohydrate residues may play an important role in regulating desensitization of transmitter receptors.