The germination of Lycopus europaeus seeds depends absolutely on exposure to light and fluctuating temperatures. Studies of temperature responses were made to establish the minimum fluctuation required for a response, the interaction of temperature and exposure time in different parts of the alternating temperature cycle, and the effects of successive transfers between cycling temperature conditions. There was a complex interaction between these three. The minimum fluctuation never fell below 6.5 °C but varied up to c. 15 °C according to other test conditions. High temperatures favoured rapid responses, and exposure to temperatures above 20 °C in one or other phase of the temperature cycle was essential for a full response. No response occurred either at any temperature under constant conditions, or if only one temperature change was given. Under some conditions a single cycle of alternating temperatures, including two changes of temperature, promoted high germination rates.