The range of entrainment of the locomotor activity rhythm of the cockroachLeucophaea maderaeis increased at both the upper and the lower limit by Li administration. This is not due to an increase of the Zeitgeber effect of light on the internal oscillators of the cockroach, as we tested by decreasing the intensity of the light of a 14:10 LD cycle. The threshold at which free run or relative coordination occurred was found to be 4.10‐11W/cm2(528 nm) and varied interindividually for 4 orders of magnitude. This threshold does not correlate with period length in DD. Adding Li leads only in 1 out of 15 cases to resynchronization, and this was due to a strong period lengthening in this particular animal rather than to an increased sensitivity of the oscillator to the Zeitgeber. Comparing populations, the mean threshold of synchronization under Li was the same as under water. If in the same individual the threshold was determined first under water, then under Li, synchronization was ameliorated. Age or after effects could be responsible for these results. Li seems to interact with coupling of oscillators.