Abstract: Adaptation of single cardiac ryanodine receptor (RyR) channels was demonstrated by application of the caged calcium ion ([Ca.sup.2+]) methodology. In contrast to conventional desensitization found in surface membrane ligand-gated channels, single cardiac RyR channels adapted to maintained [Ca.sup.2+] stimuli, preserving their ability to respond to a second (larger) [Ca.sup.2+] stimulus. RyR adaptation may represent a molecular control mechanism for smoothly graded [Ca.sup.2+]-induced [Ca.sup.2+] release in heart and may be a fundamental feature of channels, including the inositol trisphosphate receptor, that are involved in intracellular [Ca.sup.2+] signaling in many cell types.