Cardiovascular K+Channel Molecular Biology. K+channels represent the most diverse class of voltage‐gated ion channels in terms of function and structure. Voltage‐gated K+channels in the heart establish the resting membrane K permeability, modulate the frequency and duration of action potentials, and are targets of several antiarrhythmic drugs. Consequently, an understanding of K+channel structure‐function relationships and pharmacology is of great practical interest. However, the presence of multiple overlapping currents in native cardiac myocytes complicates the study of basic K+channel function and drug‐channel interactions in these cells. The application of molecular cloning technology to cardiovascular K+channels has identified the primary structure of these proteins, and heterologous expression systems have allowed a detailed analysis of channel function and pharmacology without contaminating currents. To date six different K+channels have been cloned from rat and human heart, and all have been functionally characterized in eitherXenopusoocytes or mammalian tissue culture systems. This initial research is an important step toward understanding the molecular basis of the action potential in the heart. An important challenge for the future is to determine the cell‐specific expression and relative contribution of these cloned channels to cardiac exc