Both the transforming ammocoete and parasitic Petromyzon marinus display an endogenous cyclic activity rhythm. The rhythm is gradually lost in constant conditions of dim light but is re-established by exposing the animal to one or two artificial diurnal light cycles. The period of the endogenous cycle in one larva was 22 hours 58 minutes. Rhythm in the parasite can also be re-established by exposing the animal during a single short period to chemical sensory stimulation produced by the scent of fish. Diurnal rhythm so established was maintained during a period of 5 days at the end of which the observations ceased. Sensory stimulation can function as a “setting” mechanism of the physiological clock. In this instance the setting mechanism is not related to the metabolism of the animal but must have its seat in the central nervous system.