1. In Neurospora crassa strain bd a, circadian rhythmicity was investigated in constant darkness. In the growing front of solid agar cultures, conidiation was preceded by a maximum of NAD+ kinase activity. The enzyme rhythmicity was suppressed by culturing mycelial discs in liquid medium, but became detectable again when this treatment was followed by 3 hr of air exposure. 2. In air-exposed discs, the circadian maximum of NAD+ kinase was enhanced by about 3-fold and advanced by about 3 hr upon administration of 1,2-dihydro-4-hydroxy-6-methoxy-N-methyl-quinoline (DHMMQ), whereas melatonin tended to reduce this rhythm. 3. DHMMQ stimulated the autoreduction of NADP+, the maximum of which followed that of NAD+ kinase. 4. Bioluminescence was measured in the dinoflagellate Gonyaulax polyedra, which was transferred from a light/dark cycle of 12:12 hr to constant darkness. 5. DHMMQ stimulated light emission up to more than 13-fold. When given at the low concentration of 10−6 M, the sensitivity towards DHMMQ varied greatly within the circadian cycle; the maximum of responsiveness occurred 3 hr after the onset of darkness, whereas no effects were seen in the late part of subjective night and during subjective day.