Ribosomes are absolutely essential for growth but are, on the other hand, energetically costly to produce. Therefore, it is important to adjust the cellular ribosome levels according to the environmental conditions in order to obtain the highest possible growth rate while avoiding energy wastage on excess ribosome biosynthesis. Here we show, by three different methods, that the ribosomal RNA content of Escherichia coli is downregulated within minutes of the removal of an essential nutrient from the growth medium, or after transcription initiation is inhibited. The kinetics of the ribosomal RNA reduction vary depending on which nutrient the cells are starved for. The number of ribosomes per OD unit of cells is roughly halved after 80 minutes of starvation for isoleucine or phosphate, whilst the ribosome reduction is less extensive when the cells are starved for glucose. Collectively, the results presented here support the simple model proposed previously, which identifies inactive ribosomal subunits as the substrates for degradation, since the most substantial rRNA degradation is observed under the starvation conditions that most directly affect protein synthesis.