1. Some possible molecular mechanisms of action of the anxiolytic, anticonvulsant and neuroprotective agent MK-801 have been examined in 'whole-cell' voltage clamp recordings performed on rat hippocampal and cortical neurones, bovine adrenomedullary chromaffin cells and N1E-115 neuroblastoma cells maintained in cell culture. 2. Transmembrane currents recorded from rat hippocampal and cortical neurones in response to locally applied N-methyl-D-aspartate (NMDA) were antagonized by MK-801 (0.1-3.0 microM). Blockade was use-dependent, and little influenced by transmembrane potential. MK-801 (3 microM) had no effect on currents evoked by kainate (100 microM). 3. The antagonism of NMDA-induced currents by MK-801 was only slowly and incompletely reversed when the cell membrane potential was clamped at -60 mV during washout. Prolonged applications of NMDA at +40, but not -60 mV during washout, markedly accelerated recovery from block. 4. In contrast to MK-801, ketamine (10 microM) blocked NMDA-induced currents in a voltage-dependent manner. Blockade increased with membrane hyperpolarization and was completely reversible upon washout. 5. MK-801 (1-10 microM) produced a voltage- and concentration-dependent block of membrane currents elicited by ionophoretically applied acetylcholine (ACh) recorded from bovine chromaffin cells. The block was readily reversible upon washout. 6. gamma-Aminobutyric acidA (GABAA) receptor-mediated chloride currents of chromaffin cells were unaffected by MK-801 (1-100 microM). In contrast, such currents were potentiated by diazepam (1 microM). MK-801 (100 microM) had no effect on currents evoked by GABA on hippocampal neurones. 7. MK-801 (10 microM) had little effect on membrane currents recorded from N1E-115 neuroblastoma cells in response to ionophoretically applied 5-hydroxytryptamine (5-HT). Such currents were antagonized by the 5-HT3 receptor antagonist GR 38032F (1 nM) and also by MK-801 at high concentration (100 microM). 8. Voltage-activated, tetrodotoxin-sensitive, sodium currents of chromaffin cells were unaffected by 10 microM MK-801. However, at a relatively high concentration (100 microM), MK-801 reduced the amplitude of such currents to approximately 77% of control. 9. The relevance of the present results to the central actions of MK-801 is discussed.