When barbitone was added to turbidostat cultures of Klebsiella aerogenes in the steady state of growth in a glucose-salts medium pronounced and reproducible damped oscillations occurred in the growth rate until a new steady state was established. The mean size, the DNA, RNA and protein content and the glucokinase, glucose phosphate isomerase and glucose dehydrogenase specific activities of the cells also fluctuated while the glucose-6-phosphate and 6-phosphogluconate dehydrogenase activities changed monotonically. The new steady state in drug medium was characterised by a reduced rate of growth, a diminished DNA content, a significantly augmented level of activity of all the enzymes studied, particularly glucose-6-phosphate and 6-phosphogluconate dehydrogenases where the increases were 2.4-fold. The intracellular drug concentration also oscillated in the transition phase, but in an inverse manner to the growth rate, a Michaelis-Menten type of relationship existing between them. When the growth rate was at its lowest, the intracellular drug concentration was close to the external level but in the final steady state about 90 per cent of the drug was excluded from the cells. The results are discussed in relation to barbitone-induced responses in animals.