1. In cats under Dial, thirty different anions were injected into some 400 cerebral cortical neurones, by diffusion from micro-electrodes, while observing IPSPs evoked by surface stimulation. 2. The probability of stable intracellular recording was much enhanced when micro-electrodes were filled with an organic salt of K. 3. With the exception of I03-, all univalent anions tested were capable of producing a reversal of IPSPs in a substantial proportion of cases. IPSPs therefore appear to be associated with a high permeability to many univalent anions. 4. Apart from CrO4-, multivalent anions only rarely caused a reversal of an IPSP. 5. The probability of observing a reversal with inorganic anions was a very steep function of their relative mobility in water. The relative mobilities through the membrane, calculated on the assumption that the membrane pores have the same permeability at rest and during IPSPs. are consistent with the hypothesis that inorganic anions travel through pores whose equivalent diameter is about twice the diameter of hydrated Cl- and is appreciably greater than that of the corresponding pores in spinal motoneurones. 6. The probability of observing a reversal with organic univalent anions (mainly of aliphatic and aromatic acids) bore an approximately linear relation to their mobility in water. These anions have a much higher mobility through the membrane (during inhibition, and perhaps even at rest) than would be expected from their size if they travel through the same pores as the inorganic anions. 7. As in other membranes, the rate of diffusion of various ions is evidently determined not only by the ions' size relative to membrane pores, but also by their chemical structure and their ability to interact with membrane constituents. © 1969 Springer-Verlag.
