Alcohols affect the circadian rhythm of random motility of non dividing autotrophicEuglena gracilisin two different ways:1.In the low concentration range (50–200 mM permanently) ETHANOL lengthens the period up to several hours. Simultaneously the cellular respiration is enhanced and gluconeogenesis and cell division is stimulated. The new metabolic state and the longer stationary period are achieved within a transient of 2–3 days.BUTANOL lengthens the circadian period and induces metabolic activities as ethanol but with an additional lag of two days.PROPANOL and METHANOL are not metabolized by cells in our test conditions as well as they do not lengthen the circadian period.From these results and from supplementary experiments with isolated mitochondria from Euglena we conclude that alcohols affect the rhythm via the metabolism most probably due to the activation of a mitochondrial NADH oxidase available to the cytosol NADH and coupled to three sites of phosphorylation.2.Applied in short pulses of high concentrations (30 min 1–1.5 M) ETHANOL decreases the circadian amplitude by injuring the cells to an extent which depends on the circadian phase at which the alcohol has been applied. The phase dependence for the ethanol susceptibility displays a minimum at the physiological evening and a maximum at the physiological morning. Phase and period length of the rhythm are not affected.Our results do not indicate an essential role of membranes in the mechanism of the circadian clock.