Acetylcholinesterase (AChE) activity in the brain ofCatolesandMusexhibited cyclical variations during the 24 h period of the solar day. Crest values for AChE activity at 08.00 h inCaloleswas 180° out of phase with those ofMus. Rhythm characteristics viz., M, A,ϕ(computed acrophase) and D (periodicity index) differed for the animals. Also, in both cases Ho (Null Hypothesis) was rejected (A ? o). Hence a significant and phase‐synchronized rhythm with angular frequencyω(held constant at one cycle per 24 h) was observed. These observations were correlated to motoric patterns of the animals.