Over the past decade, a variety of ion channels have been identified and characterized in lymphocytes by use of the patch-clamp technique. This review discusses biophysical and regulatory aspects of lymphocyte potassium and calcium channels with the aim of understanding the role of these channels in lymphocyte functions. Lymphocytes express both voltage-dependent potassium [K(V)] channels and calcium-activated potassium [K(Ca)] channels, and each is upregulated as cells progress toward division following mitogenic stimulation. The genes encoding two K(V) channels, Kv1.3 (type n) and Kv3.1 (type l), have been cloned. Mutational analysis is revealing functionally important regions of these channel proteins. Exogenous expression studies and the use of highly specific channel blockers have helped to establish the roles of type n K(V) channels in sustaining the resting membrane potential, in regulating cell volume, and in enabling lymphocyte activation. Blockade of K(V) and K(Ca) channels effectively inhibits the antigen-driven activation of lymphocytes, probably by inducing membrane depolarization and thereby diminishing calcium influx. A prolonged rise in intracellular calcium ([Ca2+]i) is a required signal for lymphocyte activation by antigen or mitogens. Single-cell fluorescence measurements have revealed underlying [Ca2+]i oscillations that are linked closely to the opening and closing of Ca2+ and K+ channels. Sustained Ca2+ signaling and oscillations depend absolutely on plasma-membrane Ca2+ channels that are activated by the depletion of intracellular calcium stores. Under physiological conditions these channels open as a consequence of store depletion induced by inositol 1,4,5-trisphosphate (IP3), but they can also be activated experimentally by several agents that empty the stores without generating IP3, such as the microsomal Ca(2+)-ATPase inhibitor thapsigargin. The intricate causal relationships among ion channels, membrane potential, [Ca2+]i, and lymphokine gene expression can now be pursued at the single-cell level with patch-clamp recording, calcium-dependent dyes, reporter genes, and fluorescence video techniques. These approaches will help to clarify the essential roles of ion channels in the molecular pathways subserving activation and other lymphocyte behaviors.