The periglomerular cells of the rat olfactory bulb, a virtually unknown population of interneurons, have been studied applying the whole-cell patch-clamp technique to thin slices. A prominent result, obtained under current-clamp conditions, is that these cells appear to be functionally heterogeneous, and show distinct excitability profiles. Voltage-clamp analysis allows the identification of the ionic basis of these differences and suggests a division into at least two classes, based on the characteristics of the K+ conductances. The first group displays two K+ conductances (delayed rectifier, gKV, and fast transient, gA) of similar amplitude, and under current-clamp conditions shows the usual outward rectifying behaviour at depolarized potentials. The second group has a large gA, and a small or absent gKV. Consequently, following sustained depolarizations under current-clamp conditions leading to inactivation of gA, these neurons do not show any sign of outward rectification and behave as ohmic elements, as normally observed only at hyperpolarized potentials. The transition ion zinc (10-300 microM) affects gA but not gKV The inactivation process (steady-state curve and rate constant) is strongly altered by Zn2+, the activation process less so; open-channel conductance is not affected. The Zn2+ effect is unlikely to be due to surface charge screening or to a mechanism involving channel block. In view of the substantial presence of zinc ions in the olfactory nerve terminals, its actions on the A-current could be of some relevance for physiological function.