Polyribosomes from rat epididymal fat pads may be prepared in a synthetically active state and high yield by quick-freezing the tissue in liquid nitrogen, followed by grinding to a frozen powder. The number of initiated (active) ribosomes is limiting in a cell-free protein synthesis assay derived from the frozen preparation that measures completion of nascent polypeptide chains. When starved rats are killed 30 min after a single injection of insulin (0.8 unit, subcutaneously) there is a shift in the ribosomal distribution pattern (on sucrose density gradients) in the direction of larger polyribosomes. This is accompanied by a 2-fold increase in protein synthetic activity of fat cell ribosomes as measured in vitro. When isolated epididymal fat cells are treated with insulin, with or without added glucose, a 2-fold increase in the rate of incorporation of [3H]leucine into [3H]leucine into trichloroacetic acid-insoluble protein is noted within 40 min. The parallel increases in the number of initiated ribosomes, in their synthetic activity in vitro, and in the apparent protein synthetic activity of the whole cell indicate a rapidly evolving action of insulin at the level of peptide chain initiation.