1. Whole-cell voltage clamp recordings were used to study the action of the transition ion zinc on the A-current kinetics in granule cells from rat cerebellar slices. 2. The effects of zinc have been tested in the concentration range from 1 microM to 1 mM, and fully characterized on all kinetic parameters at 100 and 300 microM. All the effects observed were rapid, concentration dependent and fully reversible. 3. Steady-state inactivation curves are strongly shifted towards depolarized potentials, with activation curves much less so. These shifts lead to an increase of the peak current amplitude around physiological resting membrane potentials and to a decrease at hyperpolarized potentials. 4. The forward 'on' rate constants are slowed by Zn2+ at a concentration of 100-300 microM by a factor from 1.5 to 4. The backward 'off' rate constants are unaffected by Zn2+. 5. The development of IA inactivation, as measured from the current decay, is not affected by Zn2+ up to 1 mM. Removal of inactivation is, on the contrary, significantly slowed. 6. The results are neither compatible with the theory of the surface charge screening effect nor with a mechanism involving channel block. It seems more likely that Zn2+ interferes with the channel gating by binding to a specific domain of the channel protein. 7. After treatment with Hg2+, which is irreversible, Zn2+ still maintains its effects, which suggest that the two divalents act at different sites. 8. In view of the widespread distribution of zinc throughout the brain, its actions on the A-current could play an important role in physiological function.