The considerable volume of data now available strongly implicates the modulation of ion channel activity as a key function of the PKC enzyme family. In some systems, such as the PC 12, RINm5F, and NG108-15 clonal cell lines, the action of the enzyme is becoming clear, whereas in others, such as cardiac and smooth muscle cells, contradictory evidence exists, and clarification of the role of PKC in these tissues must await further analysis. Few general statements can be made concerning the regulation of ion conductance through the various classes of channel, or through one particular species of channel that is expressed in and subject to regulation in different cell types. Rather, it seems that many different patterns of modulation can occur in different cells, and so, elucidation of the molecular mechanisms that determine these diverse patterns represents a major challenge. To summarize, we will focus on three aspects of the modulatory action of PKC.