Conditions are described for the preparation of functional myofibrils and myosin light chains from freeze-clamped beating hearts with the state of light chain phosphorylation chemically 'frozen' during the extraction procedure. Myofibrils were shown to be functionally intact by measurement of Ca2+ binding and ATPase activity. Highly purified cardiac myosin light chains could be routinely isolated from myofibrillar preparations using ethanol fractionation together with ion-exchange chromatography. Analysis of light chains for covalent phosphate indicated that basal levels of phosphorylation of the 18--20 000 dalton light chain of myosin in rabbit hearts beating in situ or in a perfusion apparatus were 0.3--0.4 mol/mol. Covalent phosphate content of the light chain fraction did not change during perfusion of hearts with 10 microM epinephrine.