An amplitude rhythm of 12 sec duration exhibiting a gradual decrease of amplitude per cycle was found and confirmed via autocorrelation inLimulus polphemus. This rhyth‐micity was nonexistent in the same animal 30 min later. Such an amplitude rhythm was not found inHadrurus arizonensis, although it was possible that a much longer cycle existed. The amplitude cycles and the heart beat were compared to Wever's (1965) pendulum and relaxation oscillation. It was proposed that the amplitude rhythm was predominantly dependent on the environment.