The multiplication rate and the rate of frond expansion of Lemna minor L. decline to a low level after transfer from continuous light to continuous darkness. The time course of this decline was measured. Conditions during the light period which were favourable for synthetic processes retard the decline. A retardation was also effectuated by kinetin during darkness. After 3 weeks in darkness the growth level of kinetin and non-kinetin cultures is about the same. With 5 minutes red light every 24 hours, in the presence of kinetin, a much higher growth rate is maintained. The effect of kinetin on growth in darkness is not substitutive for, but synergestic with that of short time illuminations.