We investigated the diurnal rhythmicity ofin vitrotranslated mRNAs for integral membrane proteins. Microsomes were isolated from rat liver in different phases of the 24‐h cycle. The polysomal mRNA was translated in a hepaticin vitrosystem containing tRNA, pH 5 enzyme, Sephadex G‐25 eluate, an ATP regenerating system, GTP, spermidine, controlled concentrations of amino acids, and3H leucine. After translation, the membranes were freed from extrinsic proteins by treatment with EDTA and high LiCl. Integral proteins were extracted from the membrane vesicles by SDS, and separated by polyacrylamide gel electrophoresis.3H leucine incorporation into various individual integral membrane proteins revealed diurnal rhythms in the corresponding mRNA activities.