Since Manson1 discovered the phenomenon of filarial periodicity in China numerous investigators have sought to elucidate its mechanism. Mackenzie2 was the first to reverse this periodicity by inverting the period of sleep and activity and his success led him to the belief that the sleeping and waking states are primary factors in its production. Manson successfully repeated and varied Mackenzie's experiment and since then a number of investigators have confirmed it, including Low, Manson-Bahr and Walters.3, 4 These workers pointed out, on the basis of very careful 2-hour sampling, that true reversal did not occur but an irregular periodicity resulted from the patient's changing his daily routine.In an endeavor to shed some light on the mechanism of periodicity an attempt was made to reverse the cycle by changing the daily routine of filaria-infected dogs. Two animals were chosen for this experiment, one with a relatively high number of microfilariae (minimum of 40,000 per cu. cm.) and the other with only ...