Photic information transmitted from the eyes to the suprachiasmatic nucleus (SCN) is essential for entrainment of circadian behavioral and physiological rhythms in mammals. Under conditions of constant darkness, these rhythms are maintained by the circadian pacemaker cells of the SCN [Bioessays 22 (2000) 23]. It is possible, however, that even in the absence of light, the eyes, which also contain autonomous circadian pacemakers [Science 272 (1996) 419; Chronobiol Int 16 (1999) 229], modulate circadian rhythms in the SCN. Indeed, it was shown recently that removal of the eyes abolishes an endogenous circadian rhythm within cells of the SCN [Nat Neurosci 6 (2003) 111], a finding that led to the suggestion that specific rhythms of the SCN are driven by input from the eyes. In contrast, we show here that removal of the eyes amplifies a normally dampened endogenous circadian rhythm within the SCN, indicating that the eyes can suppress the expression of specific rhythms within the SCN while promoting others.