Polysomal messenger RNA from rat liver microsomal fractions was translated in a hepatic invitrosystem containing among other components Sephadex G‐25 eluate.3H leucine incorporation was measured both in overall protein and in integral membrane proteins prepared from microsomal membranes by treatment with EDTA and 2M LiCl. In the first series of experiments, microsomes from different phases of the diurnal cycle were incubated for protein synthesis with the same G‐25 eluate from a single time of day. The rates of protein synthesis revealed a diurnal rhythmicity with major maximum at 300h. In the second series, two microsomal fractions from different diurnal phases were pre‐incubated with their own post‐microsomal supernatant and with that from another time of day. The incubation of the re‐isolated microsomes was carried out with two G‐25 eluates from different phases. These experiments indicated the existence of a diurnally fluctuating cytosolic factor or factors controlling protein synthesis. The existence of such a factor was confirmed by a third series of experiments: microsomal preparations from a single time of day were incubated with Sephadex G‐25 eluates from various phases. The degree of stimulation of overall and membrane protein synthesis exerted by the eluates was found to change rhythmically.