Melatonin administered to rat intraperitoneally organizes ultradian rhythm of protein synthesis in hepatocytes that persists for 1 day after exposure to the synchronizing signal. Hepatocytes were isolated 1 day after melatonin administration and cultured on coverslips in a serum-free medium. In 24 h in culture, the kinetics of protein synthesis was analyzed. In our previous experiments, we detected a rhythm in cells isolated in 1.5 h, but not in 3 days after melatonin administration to the rat. We have found that synchronization of oscillations of the protein synthesis intensity in vivo persists over 1 day after rat exposure to melatonin. Phenylephrine, an efficient synchronizer of protein synthesis in vitro, does not organize the rhythm in vivo.