1. In cats under methoxyflurane, DNP and other metabolic inhibitors were tested on cortical neurones by iontophoresis from micropipettes.2. DNP, dinitro-o-cresol, iodoacetate, pentachlorophenol and oligomycin (uncouplers or inhibitors of oxidative phosphorylation), as well as moderate anoxia, blocked selectively and reversibly spontaneous firing and discharges evoked by ACh; responses evoked by glutamate were facilitated by moderate doses of DNP and blocked only by large amounts.3. Azide, cyanide, ouabain and strophanthidine had a mainly excitatory effect; the cardiac glycosides tended to depress more strongly responses to glutamate.4. Intracellular observations showed that DNP causes a sharp fall in electrical excitability, associated with a hyperpolarization and fall in membrane resistance.5. The hyperpolarizing action of DNP had a mean reversal level (E(DNP)) nearly 30 mV more negative than the resting potential; E(DNP) was identical with the mean reversal level for the depolarizing action of ACh, measured on the same cells.6. DNP had its usual hyperpolarizing effect on neurones whose IPSPs had been made positive by raising the internal [Cl]; the mean E(IPSP) was over 30 mV more positive than E(DNP).7. It is concluded that DNP lowers excitability by raising the membrane conductance to K(+) (g(K)) and that it blocks ACh responses selectively because ACh has a precisely opposite action on these neurones.8. In the Discussion, it is suggested that the rise in g(K) is mediated by an increase in internal free Ca(2+), caused by a slowing of mitochondrial activity, and that a similar mechanism may play a significant role in general anaesthesia.