Relative rates of production and steady state levels of ribosomal proteins were determined in a temperature sensitive rpsD (S4) mutant of Escherichia coli. Some proteins (S4, S12, S13) were overproduced in the mutant at permissive temperature but steady state levels of all examined ribosomal proteins were normal. In a rpsD+/rpsD+ homodiploid strain the relative rates of production of ribosomal proteins were not affected by the increased gene dose. In a rpsD+/rpsD heterodiploid strain only wild type, but not mutant S4, was found. In such a strain S4, S7, S12 and probably S13 is overproduced. It is implied that S4 is involved in the regulation of expression of proximal genes of the two transcriptional units including the genes coding for S4 itself and S12, respectively. A degradation system for ribosomal proteins, which is rapid enough to be of regulatory significance, is demonstrated.