A rhythm of the [3H]leucine incorporation rate with a period of about one hour (circahoralian rhythm) has been found in rat hepatocytes grown in vitro as a monolayer and in liver organ culture. The periodicity of the incorporation rate remained after correction for changes in leucine pool size. A similar periodicity of the leucine incorporation rate was detected in a cell-free system prepared from rat hepatocytes. We have also found circahoralian oscillations of the ATP level and similar oscillations of the leucine tRNA aminoacylation rate in a hepatocyte monolayer. The addition of 1 mM ADP to the culture resulted in a considerable increase in the ATP level in the cells, but the rhythm of protein synthesis was retained under these conditions. The conclusion that there is a flexible association between changes in the ATP and GTP levels on the one hand, and oscillations of the protein synthesis rate on the other, is also supported by experiments with a cell-free system, in which the rhythm of protein synthesis rate was observed in the presence of excess ATP and GTP. We propose an hypothesis to explain the fractal pattern of circahoralian metabolic rhythms.