The properties of L-glutamate sensitive neurones in the fresh-water snail Lymnaea stagnalis were studied to determine whether or not NMDA type receptors are present in molluscan ganglia. Experiments were carried out in isolated ganglia and with dialysed neurones. In the neuro-secretory light yellow cells (LYC) both glutamate and NMDA (but not other glutamate agonists) induced prolonged depolarization and inward current. The sensitivity of the neurones for NMDA was lower than for glutamate. Both currents are carried by Na- and Ca-ions and have linear I-V characteristics. Mg-ions did not reduce either but glycine potentiated both glutamate and NMDA responses. It is concluded that in the molluscan central nervous system glutamate receptors have some of the characteristics of mammalian NMDA receptors.