The accumulation of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and protein was followed in cultures of Escherichia coli B/r during exponential growth in different media and for 2 h after a nutritional shift-up from succinate minimal medium (growth rate [mu1] = 0.67 doublings per h) to glucose plus amino acids medium (mu2 = 3.14 doublings per h). During postshift growth of the culture, the amounts of RNA (R), DNA (D), and protein (P) increased such that the ratios of the increments (delta R/delta P; delta D/delta P) were constants (k1, k2). This implies that the rates of accumulation of nuclei1:k2:1. These constants change from their preshift value to their final postshift value (i.e., k1 and k2) within a few minutes after the shift. k1 is a function of the activity of ribosomes, whereas k2 is related to the initiation of rounds of DNA replication. These parameters and the observed change in the doubling time of RNA (= mu2/mu1) were used to derive kinetic equations that describe the accumulation of DNA, RNA, protein, and cell mass during the 2- to 3-h transition period after a shift-up. The calculated kinetics agree closely with the observed kinetics.