The effects of exposure to various photoregimes or to constant darkness on liver glycogen and plasma glucose levels in the goldfish were examined during different seasons. In addition, pinealectomized fish and optic nerve sectioned fish were exposed to the photoregimes to assess the potential role of the pineal organ and retinal pathways in mediating photoperiod effects. Neither photoperiod manipulation nor exposure to constant darkness affected liver glycogen levels in experiments conducted during October and January. Photoperiod alteration and exposure to constant darkness resulted in changes of liver glycogen and plasma glucose levels during May, suggesting that response to photoperiod varies with season. Likewise, optic nerve sectioning altered liver glycogen levels during May, but not during October or January, indicating that retinal pathways may be involved in mediating photoperiod effects. During both October and May pinealectomy resulted in a lowering of liver glycogen levels independent of photoregime ; the extent to which hepatic glycogen levels were decreased by pinealectomy was affected by lighting conditions. Plasma glucose levels were elevated in pinealectomized fish regardless of photoregime during May ; the degree of this increase varied with lighting conditions. Our data suggest that the pineal organ does not mediate photoperiod effects on plasma glucose and hepatic glycogen levels, but that there is a functional relationship between this organ and carbohydrate metabolism.